On Thu, 8 Oct 2020 11:49:17 +0100, Roland Perry wrote:

Currently I have a boiler with a controller that simply turns it on (for
CH) during pre-programmed hours. Rads have thermostatic valves.

So presumably the boiler short cycles when the timer says on but the
rad valves have all shut, pushing up your energy bill...

I'd like to add a room thermostat in the living room [running additional
wiring isn't an issue] but need to know if it can simply be put in
series with the timer-control.

That should work boiler will only fire when the timer is on and the
'stat says cold. However I'd fit a programable thermostat rather than
the simple on/off kind.

It'd wire in just like a normal stat but the boilers heating time
switch would be set to constant giving the programmable stat control
of the temperature for each time period. Programable stats often
preempt the first target temperature of the day. Turning the boiler
on just early enough for the target temp to be met at the set time.

I realise that the heat in the rest of the house will suffer if the
living room is warm enough, but surely that's a natural consequence of
any room thermostat.

It would be more usual to put the room stat in say the hall and set a
little low (as a hall is generally cool) so it knocks off the heating
when the rest of the house is up to temperature. Which ever room the
room stat is in it should have a radiator with its thermostatic valve
fully open or disabled. Otherwise the two stats may "fight".

--
Cheers
Dave.