Thanks for your help Carpenter. I'll take pictures this evening. I checked out homebond on the internet as well and then rang the builder again. I told him the wet stain was spreading (which it has done by about 3 feet over the weekend) and told him I wasn't happy at all. I told him that I was on to the surveyor who did the snag a few months ago and that it had a "homebond guarantee with 5 years smoke and water..." and at that point he says they'd take down the ceiling again and find out what's happening AND fix us up with a few bob for a damp stain on the carpet too.
Thanks too, ludermor. You're right and I really didn't want to go down that route but couldn't think of what else to do (I know shag all about the industry). Well it seems that subtle threat has done the trick. If nothing happens though I'll do a letter like Carpenter says and go up the chain of command like you said.
Thanks again, I'll let you know how it goes.