On 12 Dec, 14:41, Calvin wrote:
Top of my checklist would be a failed zone valve motor. *If it's a
system with 2-port valves you might find a little lever on the valve
body which manually opens the valve. *If you do it should move very
very easily when the system is calling for heat (ie ON) and quite
slowly as you spin all the gears when the system is off. *If it's
stiff in both cases the motor has failed which is not unusual (I keep
a spare in my DIY kit) and easy to fix. *If that has happened you can
get heat in the short term by latching it open.

Sounds plausible, thanks. I'll go take a look. And then get on the
phone to rant at the bloke who supposedly serviced the boiler (he
thought the heating problem was the TRVs on the rads sticking, which
sounds like nonsense as some are new and all rads have the same
problem and got it at the same time).

Jon