I've never used Fasthosts, but most hosts will generally make it a pain in the arse to cancel their services (although some are better than others) Sometimes a phone call will sort it out, or even just raising a "ticket" in their support section.
I can't remember who it was, but a few years ago I wanted to can the hosting package and they just wouldn't play ball, so I just canceled the credit card I was using and got a new one. They can't take money from a nonexistent card!