Hi,
Just thought I would let you know what I have done, Purchased from B&Q they sell 1 Inch box section in metre lengths @ £8.74, two ways I have done this firstly take out the seat bottom and measure from the lip at the bottom of the bulkhead to front of seat box and cut, drill two holes in the box section where the seat already bolts and put it back together, this give me an extra two inches travel and raises the seat by an inch which also lets you tilt the seat back a little more. Second method is two cut the metre section straight in half and remove a 1 inch piece on the the lip of bulkhead, this allows for the box to be mounted an inch further back giving you another inch of travel backwards, but does restrict the tilt on the seat back if you fully use the travel.
Cheers Paul