OK, what's up in the loft?

[Cubist]

Mice tunnel under the insulation and run about on the plasterboard beneath. I had these ticky scrabbling noises in my previous house every winter, and in this house until I found the little hole in the brickwork where they were getting in and shinning up inside the wall cavity. Once that was blocked up, no more mice.

That sounds like confirmation of my suspicions. Good oh.