In message , Sacha
writes
IME they live in walls, under terraces etc. and are almost impossible
to get rid of. That said, while they're not pleasant to see around the
kitchen in any great numbers and they can nip, if disturbed, they don't
spread disease or do any real harm that I'm aware of. In my early
days on urg, I seem to remember someone suggesting sprinkling Vim or
Ajax powder (can one still find it?) around the places they travel. I
tried that and it did seem to work.

Yes, all they need is a tiny crack through to something like the cavity
wall. We had never noticed them, then one year they suddenly swarmed and
flew from behind a window frame: it was a bit like some horror film. But
that was it, nothing since then - they probably noticed their mistake
and found the way to the great outdoors....

--
Klara, Gatwick basin