Thanks for the comments and the links. I've come across strategyzer.com before and I can remember thinking it was worth bookmarking – but of course I didn't.

It's not so much the problem of finding a business model I'm struggling with – it's not knowing when to run with an idea as opposed to sitting on it and looking at other avenues. I think it might be something you learn through experience (and failure). I was just hoping to see other peoples view on it.