On Saturday, 9 November 2019 12:59:42 UTC, Graham. wrote:
If the thermostat or programmer is hard wired and switching mains,
surely it would be mains driven, and wouldn't need a battery
other than for power fail backup?

Not always. Many modern ones are volt free contacts so there is no connection between the relay contacts and the internal gubbins, which are battery powered.

Means there is no requirement for a neutral at the stat location, just L and Call.

Owain