In message , at 08:39:55 on Fri, 9 Oct
2020, Roger Hayter remarked:

PS. An important point about modern room thermostats. 98% of easily
available ones have a tiny heater to reduce hysteresis and need a mains
neutral and live connection, and *must* be connected the right way round, it
matters which is the mains supply side and which is the switched side. (It
actually doesn't matter which is neutral and which live.) If you are not
sure if your controller is mains or SELV from the boiler you need to find a
so-called two wire room thermostat which does not need a neutral connection or
240v for a heater.

Thanks for that. So I've ordered a 2-wire one from Screwfix.

Sufficiently unusual that none of the local branches have it in stock;
will arrive in a couple of days.
--
Roland Perry