Man up!
All MK1 clutches are stiff.
there is a "volvo soft pedal conversion kit"
Maybe Volvo 480 iirc?
You can't just use a longer fork, you'll need the bracket to lift the clutch cable aswell as the larger pivot bush. Parts can be 172 or Volvo, Volvo part numbers are kicking about somewhere but I don't have them to hand.
Worth noting that you'll need to take the gearbox off in order to change the fork and pivot bush.
Oh, and yours is generally known as a 'Hybrid' rather than a Williams as the shell is a 16v.
I am extremely jealous of your numberplate!!!
P.S soft pedal conversion are for faggots.
mine was proper stiff when an aftermarket cable was used as they are slightly shorter and also only last about 6 months, I bought a Renault item and all was well.