Dead mouse or similar? No... that smell won't last too awful long... unless it's a dead moose which tends to linger longer.

The smell you describe makes me IMMEDIATELY think it's cat pee.

"I've since had the floor sanded and varnished." Soooo, is it hardwood flooring? I hope it is because replacement of the offending odrourous flooring will be much easier.... if it is cat piss, the only thing other than removal of the flooring that would work is a match.