The reason they chirp in the night?
The temperatures drop.. so battery output falls
If you manage to ignore it until the morning.. the heating has cut in and it stops chirping
Aaaaah, light bulb moment. Yes we have our heating set to go off overnight. Your explanation is obvious, I just didn't think of it! Modern house with thin interior walls and a wife who is a disturbed sleeper so not the slightest chance of ignoring it!