I just realised I didn't tell you the source of the smell. We found we had a dead-some-time mouse in the trap.
We've been aware that we've had mice in the attic for a while; they've been nibbling all sorts. Including the tube of mouse killer.
Husband has tried humane traps, poison and latterly old-fashioned mouse traps but had no success until now. I just hope the odour of death deters them.