| Adjustable rearsets?
Other than that, why not get ones of those strap-on (oo-er!) pillion Gel seats for about 30 quid, as this will not only stop you getting a numb bum but it may also raise the seat slightly to give you more room?
I think you can get them from the usual accessories places - MPS, M&P, Busters, etc. | 
antonye
Threaded Mode