Your roof void could be harbouring them. Did you have birds nesting in your
eaves or on the roof itself?

Ugh, don't tell me I'm going to have to venture in to the loft? It's
dark up there!

I have to say, I don't look up there much. I haven't noticed any
accumulated droppings on the floor (that's a sign of a nest, right?)

Last year, a pidgeon kept fluttering past the bedroom windows, I
wondered if it was nesting. Is that too long ago to be significant?