It happens that newshound formulated :
I can see the one from next door, and usually at least three from across the
road. They are well down on signal of course. I have no reason to suppose
they cause significant interference, I get much the same speed cabled to the
main router as on WiFi through the extender.

Using a Wifi scanner application, I see around 14 AP's using my laptop
on the ground floor. Two of them I believe from next door, which are
logged as stronger signals than my own, even one just 8 feet away in
the same room.

Doing the same scan on the top floor, I can get a few dozen AP's
logged.

What puzzles me, is that the signal strength graph show the usual
slight variations, but occasionally shows a massive spike in the signal
strength of some of the signals. It is perhaps an anomaly of the
scanning software.