Thanks, that's helpful. I've had thermal imaging done previously and it didn't show any major areas, just the usual door/window frames (which I'm sealing up). It feels like a more fundamental issue, but I don't really want to spend £££ or risk cavity wall..
From research it looks like Stormdry could be useful for single skin walls, but I've not seen whether it would be much use on cavity walls - does anyone have any thoughts? We don't have any damp issues internally, but the walls/house is very cold (and I don't want to go down the...