John wrote:
I have one of the small square ones from Screwfix fitted at my
daughter's house.

Had a anguished call to say that the heating was stuck on and had
reached 28 degrees.

Fiddled around and eventually swopped the batteries. It now works.

Was it due to the fiddling with all the settings - or do the contacts
'fail' in the closed position if the batteries weaken?

Thats not normal behaviour for the types I've seen.
Its usually low batteries = no heating.
Surely the opposite would not be good idea?