Hmm, it's a hard one... I used to work at a computer store and I could understand a labour charge for testing of parts, but there's a matter of good-will involved there too.
Best bet? Department of Fair Trading, just give them a call tomorrow and tell them a *little* about the story; you've checked some other parts and believe it to be this part, but to diagnose and say 100% it's that, and that it'll be swapped under warranty, they want $100. See what they say?