Now I see Ultrasonic Mouse Deterrents that just plug into a nearby
socket (I have several in the loft) and claim to affect a large area.

Use correctly ultrasonic repellers do work. I had a severe problem in
lofts in a rural area. Rats were coming in and nesting. I could kill
them with poison but then they died up there and stank.

About four years ago I fitted a Lentek PR12 device in one loft and the
problem ceased. I occasionally get a footfall but it soon goes and
doesn't come back. I have now fitted them to all roof areas and have had
no further problem. The Lenteks also send a signal through the mains
wiring. I wondered if this would interfere with equipment but it doesn't.

The key seems to be placement. Put them high up at one end so they flood
the whole volume. Make sure there is nothing producing a shadow. They
even effective through glass-fibre insulation. I have about 20 cm.

Peter Scott