Very helpful man from Pest Control visited us this morning. He is certain we had a MOUSE
Good news though that it wasn't a rat
Lack of droppings, and the fact that the poison/traps haven't been disturbed for several days makes him think it was just a single field mouse, and it has already left the way it entered.
He thinks it got in through an air brick leading to the underfloor void - apparantly they can get through any hole that is 6mm or more. I need to find a way of blocking these so that mice stay out but air gets in
He also recommended I use wire wool to block up the route from my kitchen to the loft - wire wool is easy to shove into the space and shape to its contours, is removable later if necessary, and mice hate it
Nice helpful man - free service because he didn't need to provide any treatment; just advice. He even suggested I give him a call in the summer and he will come out and deal with my wasp nests.