bbc I sympathise! When I moved into where I am now, I had a bad case of smelly mouse. Because I had just moved in I wasn't sure what this strange, sweet, sickly smell was...I also had leaky pipes and rotten floor boards, so just thought it was a 'damp' and 'falling apart' house smell.

After weeks of building works the smell was still there and worse. Eventually the kitchen cupboards were ripped out, and there it was, under the sink one, behind the facia at the bottom. It looked like a furry grey lump of soap, only recognisable of having breathed by the stench of rotting flesh. It would have stopped smelling once all the fat had disappeared and there was just bone left....according to the really nice builder who removed it.

Try and pinpoint where the smell is strongest and have a dig around.
But yes, a cat is probably the way to go. Good Luck!