Clare, these are good points, except that we tried the ultrasonic units
years ago, and they made no difference whatsoever.

I am now wondering if I shouldn't simply install some sort of vent-pipe
through the wall. Just the smallest amount of fresh air from the outside
would cure this problem, I am quite sure. Perhaps a six inch pvc pipe, with
a mesh screen to defeat animal (more mice!) entry....

I bet there is something on the market that would work, I will look
around....

James