I've just fitted a new clutch slave cylinder piston, and obviously had to bleed the clutch afterwards - in fact I just replaced all the fluid. It was only after I'd finished that I noticed I'd picked up DOT 5.1 rather than DOT 4 in Halfords.
I'm not too well up on these things - hope it's gonna be OK (ie: won't melt the seals or anything like that) -