I refurbed ours: remove the inner lining by unbolting the clips at the front and easing out the plastic pegs at the bag.
Remove the old fabric: you'll probably be greeted by the manky old foam rubber which granulates with age. Remove the remnants from the liner: be careful as it's quite fragile.
Gentle wire brushing works.
Select your material: I used black leatherette, but the choice is yours. Available from upholstery/curtain places. A linear yard/metre is ample: cost about a tenner.
You can use upholstery foam but I didn't bother. The rest is down to your upholstery skills really.
Tips: first time I did it I used spray contact adhesive: due to thermal cycling (I think) it didn't last long and Sue ended up like the Black Widow, draped in black leather.
I then used old fashioned Evo Stik in a tin. Fine so far.
Be generous with the overlaps onto the back of the liner.
If using leatherette pick a warm day or do it indoors.
You will probably knacker the plastic push pins that hold the back in place: available from Halfords in the odds and sods dept.
The holes for the bolts and pins I burned through with a soldering iron.
Total cost about 18 quid.
Hope this helps.