FREE SHOULDER MOVEMENT HARNESS SYSTEM FOR BACKPACKS Field of the invention The present invention relates to backpacks in general and specifically to a shoulder strap harness system which allows free shoulder movement. Background and summary of the invention A backpack is designed to carry a load on the human body. The load is transferred to the wearer's body via a set of shoulder straps and a hipbelt. As the wearer walks, climbs, skis, skates or runs, the shoulders need to move freely so that the wearer's movements are not restricted. At the same time, the backpack must remain stably locked in position for the wearer to maintain balance. Backpacks that have shoulder straps fixed directly to the backpack will greatly restrict shoulder movement. When the wearer tries to move his shoulders, the shoulder straps will resist this movement-to move he or she has to overcome this resistance by moving the entire load-this compromises stability and hinders performance in high motion sports. Various systems have been described to allow shoulder movement in backpacks. My GB patent 214048 1B showed a method of interlinking the shoulder strap ends via a web running through the bottom of the pack. This interlinking web ran through either one or two loops in the bottom of the 1 pack folding over as it changed angle running through these loops. This method of running through loops created significant friction in the movement of the interlinked web reducing the ease of movement of the shoulders. The advantage of this system was that the interlinking web automatically exited from the loop or loops at an angle such that the web curved in one plane only between the loop and the lower end of the shoulder straps, irrespective of the height of the shoulder strap ends. This automatic adjustment of the plane of curvature of the interlinking web strap allows wearers of different backlengths to use the system with no further increase in friction in the movement of the interlinking web. My NZ patent 506106 describes a system whereby the lower ends of the shoulder straps are interlinked by a web that runs through a channel of material located between the bottom panel and the side and front panels of the backpack. Here the panels are constructed in such a way that the interlinking web curves in one plane only. The channel has a stiffened low friction surface against which the interlinking web slides. The advantage of this system is that the interlinking web can move freely through the slot allowing free movement of the shoulders. However, this is true only when the end of the shoulder straps falls in line with the plane of curvature of the bottom slot. For people with long backlengths, the shoulder strap ends will lie above the plane of curvature of the bottom slot. This means that the flexible interlinking web will be pulled into the upper edge of the channel increasing the friction and thereby inhibiting the free movement of the shoulder straps and therefore the movement of the shoulders. For people with short backlengths, the interlinking web will be pulled into the lower edge of the channel also increasing friction and free movement of the shoulder straps and therefore of the shoulders. The same limitation applies to a system I invented and marketed in the USA commencing 1993 that is described in US patent number 6,070,776. Here the interlinking web runs through a channel located in the bottom panel of the backpanel. Here also the interlinking web will only slide completely freely when the shoulder strap ends lie within the plane of curvature described by the channel in the bottom panel. For wearers of short or long backlengths, the interlinking strap will be pulled into the top or bottom edges of the channel respectively, increasing friction and reducing the free movement of the shoulders. The purpose of this invention is firstly to provide a system whereby the movement of the interlinking part at the end of the shoulder straps remains free and unrestricted for wearers of any backlength. Secondly, it is to increase the freedom of movement available to the shoulders over prior art. The pack bag comprises a bottom panel, a top panel, two side panels, a front panel and a backpanel. The backpanel is defined as the surface that touches the wearer's back. 2 The shoulder harness consists of a pair of shoulder straps that join together at their base forming a generally V shape when laid flat. The harness fits to the wearer's body as follows: the base of the V is positioned in the middle area of the upper back of the wearer. The middle of each shoulder strap passes over the shoulders and the ends of the shoulder straps lie either side of the wearer's chest. The base of the shoulder harness can preferably be attached at different heights to the backpanel to allow wearers of different backlength to match their dimensions. The middle section of the shoulder straps preferably has a web fixed at a point near or adjacent to the top of the wearer's shoulders. Each web is threaded through a buckle which is fixed to the top of the backpanel either side of center by a web strap. These buckles allow the distance between the top of the backpack and the wearer's shoulders to be adjusted for a close fit. To the ends of the shoulder straps a pair of buckles are attached. Through these buckles webs are threaded. The lower ends of these webs are fixed to the corresponding end of a smooth and flexible plastic member. The bottom of the backpack is composed of an inner and outer surface with an opening at either side. The flexible member passes through the openings and is free to slide within the gap between the inner and outer bottom panels. There is no channel restricting the lateral movement of the flexible member. The flexible member therefore forms a continuous sling under the base of the back to transfer load from the pack onto the shoulders. As the wearer moves one shoulder up and the other down, the flexible member will slide freely through the bottom panels to facilitate this movement. The width of the inner surface of the bottom panel from the front to the back edge is preferably greater than the corresponding width of the outer surface, so that the inside surface assumes a concave curve in relation to the interior of the pack. Preferably, the concavity is greatest at the back edge of the bottom panel. The flexible member may be rectangular or circular in cross section. If the member has a rectangular cross section, it is laterally stiff but longitudinally flexible and therefore free to bend easily in one plane. The angle of the plane which the flexible member takes due to the pull on the shoulder strap will automatically adjust to the height of the shoulder strap ends by resetting itself to the optimum position on the concave surface of the inside bottom panel. Thus the movement of the flexible member, and therefore the movement of the wearer's shoulders, will remain free and unrestricted, no matter what the height of the shoulder strap ends relative to the bottom panel. This system therefore allows wearers of different backlengths to be accommodated without compromising the freedom of shoulder movement. In the case where the flexible member has a circular cross section, it will be free to bend in two planes. In this case, it is not necessary that the inside surface of the bottom panel is concave in relation to the interior of the pack. The flexible member will easily bend in two places if necessary to adjust to wearers of different backlengths. 3 Stiffening rods, frames or panels are positioned at the sides of the backpanel and extend to the base of the backpanel. These prevent the rear surface of the backpanel collapsing under load. Stiffening may also be incorporated in the bottom panel to increase rigidity. The movement of the shoulders is further facilitated by two alternative enhancements of the invention. In the first enhancement, the backpack has a pair of frame stays located in the backpanel, positioned in a generally V configuration. A part of the upper section of the stay is exposed. A connector piece is attached to each stay that is free to slide up and down. Each connector piece links to the middle section of the corresponding shoulder strap via webs and buckles. When the shoulders of the wearer move up and down, the connectors slide up and down the exposed stays to facilitate this movement. For wearers of different backlengths, the connectors will automatically slide to the appropriate height for the wearer. In the alternative enhancement, the webs that attach to the middle portion of the shoulder straps are joined to buckles which in turn are fixed to the ends of an interlinking web. The central section of this interlinking web is connected to the top section of the backpack via a loop. The loop is fixed to the top panel of the backpack. The interlinking web folds and passes through this loop in a V configuration . When the shoulders move, the strap slides through the loop to allow the movement. This system allows twisting of the shoulders relative to the hips, as well as sideways bending of the shoulders. These two embodiments enhance the movement possibilities of the shoulders to full articulation. Brief description of the drawings The invention will now be described with reference to the following drawings: Figure 1 shows a side view of a pack according to the invention. Figure 2 shows a cross section through the center bottom of the pack shown in Figure 1. Figure 3 shows a side view of a pack incorporating an alternative version of the invention. Figure 4 shows a front view of the harness shown in Figures 1 and 3 on a wearer showing the movement of the shoulders and resulting movement of the harness webs. Figure 5 shows a back view of the pack. Figure 6 shows an alternative version of the back view of a pack shown in Figure 4. 4 According to Figure 1, a backpack has a lid 1, a front panel 2, side panels 3, a backpanel 4 and a bottom panel composed of an inside surface 5 and an outer surface 6, shoulder harness 7 and a hipbelt 8. The shoulder harness is linked to the bottom of the pack as follows: At the ends of the shoulder harness, buckles 9 are fixed. The top end of webs 10 is threaded through this buckle allowing the proportion of the load on the shoulder straps and hips to be varied. The bottom end of webs 10 are fixed to webs 11 and to flexible member 12. The lower end of web 11 is fixed to the edge of outer bottom 6. A slot is formed between outer bottom 6 and inner bottom 5, spanning the width of the bottom panel with openings 13 at either side of the pack. Member 12 is free to move horizontally in this slot. Flexible member 12 has a smooth surface, rounded edges, a rectangular cross section and is preferably composed of a low friction plastic material. Flexible member 12 curves upwards towards its ends. The curvature of flexible member 12 is in line with the angle of webs 10, such that flexible member 12 and webs 10 curve in one plane only. When one shoulder of the wearer is raised and the other lowered, flexible member 12 will slide freely to allow this movement. Webs 11 provide a stop to limit the range of movement just enough to allow the pack to be put on easily. Figure 2 shows a cross section of the bottom portion of the pack shown in Figure 1. At the bottom, the inside surface 5 is an approximately concave shape in relation to the interior of the pack, being wider from front to back than outer surface 6. Preferably the region of greatest curvature of inside surface 5 is towards the rear surface. Flexible member 12 will position itself on this concave surface along the line of least resistance. The position of this line is determined by the plane of curvature of flexible member 12 and webs 10. The arrows show how the plane of curvature varies for wearers with differing backlengths. Whatever the plane of curvature, flexible member 12 will automatically position itself along the line of least resistance. Figure 3 shows a pack which has a flexible member 14 with a smooth surface, a circular cross section and is preferably composed of low friction plastic material. A connector part 15 joins flexible member 14 to both webs 10 and 11. In this configuration, the inside surface 5 of the bottom panel may be the same width from front to back as the outer surface 6. Because flexible member 14 is circular in cross section it can bend with equal ease in two planes, thus it will accommodate changes in shoulder strap height due to wearers of different backlengths using the pack. Figure 4 is a front view of a wearer with a pack on. The arrows show the direction of movement of buckles 9 and webs 10 as one shoulder is lifted and the other is lowered. Figure 5 shows a back view of a pack with the hipbelt not drawn for clarity. The backpack has a pair of frame stays 16 located in the backpanel. These stays are preferably positioned in a generally V configuration being centrally located at the base of the backpanel and laterally located in the upper 5 area. These frame stays are held in position by fabric or web sleeves in the bottom and middle sections and at the top ends. The base of the shoulder harness is fixed to the backpanel behind a breathable outer backpanel suface 17. The shoulder harness has webs 18 fixed to the central portion. Webs 18 are threaded through buckles 19. Buckles 19 are linked to connectors 21 via webs 20. Connectors 21 are linked to and free to slide up or down the exposed portion of frames 16. For wearers of different backlengths, the height of connectors 21 will automatically adjust to the optimum position on stays 16. As the wearer moves his shoulders up or down, the connector will slide on the stay to allow this movement. Alternatively, the buckles 19 may be of a type which can be directly linked to frame 16 without the need for webs 20 and connectors 21. Adjusting webs 18 in buckles 19 allows the distance from the top of the pack to the shoulders to be varied. An alternative enhancement is shown in Figure 6. Buckles 19 are fixed to opposite ends of web 22. The central section of web 22 passes through a loop 23, folding over as it passes through the loop in a V configuration. Loop 23 is fixed to the top of the pack via web 24. As the shoulders move, web 22 slides freely through loop 23. This system allows twisting of the shoulders relative to the hips, as well as sideways bending of the shoulders. 6