Thanks guys.
I bought what looked like a replica part from Amazon. It had the same bore and dimensions, and looked as though the threads were the same - They weren't but as it was the same bore I thought I would try putting the piston from the new part into the old housing. It fitted really nicely. I bolted it all back in to the car and tried to bleed the system but don't seem to be having much success yet. When the pedal is pushed air is coming out of the slave cylinder bleed nipple but when it is released it seems as though air is getting in somewhere (I close the nipple before releasing the pedal). I'll take it all apart again tomorrow and see if I can find out what is wrong.
