On Tue, 21 Jul 2020 19:35:51 +0100, ARW wrote:

Is there any line of sight path between the two properties?

Yes. 20m from my router to their back door. Window my side to wall their
side.

With your router in your window the signal might get through the
wall.

Best to do a RF survey. Can your phone, when in the neighbours house
behind the wall: See your AP? How strong is that signal relative to
others on the same channel? What What other channels are in use? What
are the signal strengths there? Want to pick a channel(*) where your
AP is one of the stronger signals.

(*) On 2.4 GHz only channels 1, 6 & 11 don't mutually interfere with
each other. If there is a strong signal on one of the other channels
it may well be worth picking 1,6, or 11 that is furthest away from
it.

--
Cheers
Dave.