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

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.

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.
--
Roland Perry