Big sympathies, we tried to combat ours using generally available poison but eventually resorted to the concil rat man - absolutely fantastic! Our house has obviously had a longstanding rat history - it used to be a barn, the walls are 2 fot thick and the rat man said there will be ancient rat runs within the walls from underground so blocking entrances is not an option. He used much larger bait than we could buy (it was pink grain), inspected our whole property and land and came back 3 times untill they were gone. Think we have a dead one up there from the smell, but trust it will just desicate over time (I did find a desicated old one up there when I was up last). I have no intention of looking for it (we too have very restricted access and a lot of insulation - layers and lyers of it!) or replacing the insulation unless very bad things will happen if I dont? (I can cope with a damp patch Vs emptying the loft and reinsulating!). Our water tanks are very sealed luckily. Anyway - Council chap every time for me - £37 well spent. Good luck