I dropped lucky and bumped into a former R1150GS owner when buying a tyre from Mike at Cardiff Motorrad. £150 later I was riding away with a Sargent seat fitted to my bike. It even has the correct colour trim!
After 2600 miles riding to Austria & back I can confirm that the seat is much better than the standard seat fitted to a W reg bike. The comfy seat made the whole trip much more enjoyable. I only wish I had splashed out on one when I bought the bike in 2001...