Common Law that it is tenants responsibility to hand keys in at end of an agreement, or have to pay more rent?

You may want to look at your leasing agreement one more time. Anything that isn't in there I'd fight. Also tell them you waited for someone to show up to collect the keys, and if they say they sent someone, have them provde doentation of that person going out. Anything they are going to try to hold you accountable for HAS to be in that leasing agreement, because, on the other hand, you can very well tell the courts they never advised that the keys had to be in. Furthermore, the locks are ALWAYS changed whenever a tenant moves out, so you not turning in the keys shouldn't be that big of a deal, as they are only going to go into the garbage.