from which I deduce that the boiler /needs/ a stat and is low volatge ?

Nope. You may deduce no such thing from that statement.

P.S. A bimetallic thermostat REQUIRES a 230V connection, or it will work
very badly...

A "volt-free contact" requirement from the boiler will mean that you need
some sort of electronic stat with built in batteries. Almost all
programmable (and many electronic) thermostats meet this requirement.

Old style bimetallic thermostats have a 3 terminal connection and use 230V
power from the programmer to run a tiny heater that gives much more accurate
control to the bimetallic strip. This heater will not function as intended
on a low voltage supply and the temperature stability will be all over the
place.

Christian.