It was a dead possum in the wall cavity  @TAB  -  as@tonys said  - possums are not mechanically capable for running around in roof void and stink to high heaven when they die - and those smells move around on air currents - I had my front and back doors open when I cooked my dinner and blow flies were deafening trying to get through the screens 

All over now  - thank goodness and the rats have moved out too - I've had mice and cockroaches in the past  - not pleasant either 

I miss my cat - I would love another one but I'm past managing a pet 

All the best Tabby Cat

Owlunar