In reply to earlsdonwhu:
If it's a dual lever then take the little cap which is on top of the lever off (if it's a single lever then turn the stop valve off and pull the lever up), under that you'll find a small screw, undo it and that should allow you to remove the lever. under this you'll find a cog which should have a nut around it, undo that nut and pulling on the cog you should be able to take the cartridge out. Take that cartridge to a plumbing shop and ask for one the same type and replace it.
That'll be 120 quid.
You're welcome.