The house of a friend is fully centrally heated but none of the
radiators are fitted with thermostatic control valves. The system was
fitted about 15 years ago, possibly longer.

The house has three bedrooms and a bathroom upstairs and the
downstairs comprises a hallway, large lounge, dining room and kitchen.
All spaces are fitted with radiators including the hallway. The
system is controlled by a simple thermostat in the lounge. Hot water
is by an indirect cylinder.

I was thinking the most appropriate plan is to install TRVs to all the
radiators except those in the lounge, where the thermostat is sited,
and to change the thermostat to a programmable type.

I will appreciate guidance whether this plan is sensible or whether
all radiators should be fitted with TRVs.