In message l.net, at
12:43:37 on Fri, 9 Oct 2020, Dave Liquorice
remarked:
On 9 Oct 2020 08:39:55 GMT, Roger Hayter wrote:

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,

Bi-metal sensor based room thermostats have a heater and require a
neutral. Many modern ones use a thermistor as the temperature sensor
and don't have a heater but do need a power supply... However that
might be a battery even for a wired stat as the to be a "plugin
replacement" for a bi-metal stat it has to cope with not having a
permenant live.

I'm not 100% sure what wiring the OP has, there is mention of two
wire and two wire plus L and N.

controller /------make-------\
| | | |

N L 1 2 3 4

| | | |

from to to
mains boiler boiler

But the more I look at it, the more I suspect that either 1 or 4 is also
tied to one or other of the N/L mains. Some testing required, because
the documentation is ****.
--
Roland Perry