On Sat, 02 Jan 2021 16:44:26 +0000, Cliff Topp wrote:

We are finding that 18C on the old stat/programmer in the hallway was
more than comfortable for us but the new stat/prog has to be set to at
least 20C or 21C to 'feel' as warm as it was before.

Different thermostat, different mean temperatures, that isnt unusual.

On top of that, and perhaps even more importantly, is the frostat.

The new installation *does* have a (Honeywell) frostat wired in and it's
keeping the heating on all night, irrespective of if I turn the hallway
stat/programmer down to a very low level or completely off. This is
really bad in three ways:

It sounds like it has been wired wrongly, and the frost stat is calling
for heat by opening the CH valve. It may be something to do with the
plumbing pipes too, there should be bypass piping to allow the boiler to
run without having the CH valve open.
You clearly know nothing about it, and it sounds like your fitter didnt
either, so probably time to get someone in who knows what they are doing.