From starting out like milk that had gone off- Que blaming the 4 yr old, to a strange vomit type smell after a couple of days, to now an OMFG that's utterly disgusting WTF- smells like the worst cat shit you have ever smelt/bacterial badness..
The wall has been identified, naturally behind the hob and kitchen units, with glass splash backs and wall hung units, the worst area being above the wall units, and the extractor vent has been checked and is all ok.
I have pulled the lower panels and checked out the fireplace- the kitchen was previously 2 rooms and this end meant covering over an old fireplace but the smells not coming from there.
I cant remember the smell of decomposing animals but I imagine it must be pretty close to this, short of physically removing the units and splashbacks and knocking holes ouf of the existing walls- its an old house- circa 120 yrs, so the walls are solid granite with a lath and plaster wall with a void to let the house breath, i don't see that i have any other options but to ride it out and hope whatever is back there dries out soon enough!
I also went under the floor last night and no evidence of any rodent activity and also zero presence of the smell.
Any other ideas, other than kill everything with fire obviously!
This has been going on for about 7 days or so with the last 3/4 days at the WTF stage
and a rabbit in the last year. He looks SO happy doing it aswell... love him to bits! 
