Actually way more than one! They've (apparently) got in via a broken drain and a hole in the floorboards, and are getting behind the kitchen cupboards, up the cavity wall and into the flat roof space. They've been throwing tap dancing parties up there and I thought that was bad, but now we've had pest control round and the dead rat smell is worse than the scuttling 
We're waiting on a (very slow) process to renovate kitchen and fix the drain but it could be months away and I can't live with this!
Does anyone have any survival / smell control / horror management advice?