Heating problem

Stuck motorised valve probably. Or was it trying to heat the CH tank instead?

Yes. I'd forgotten about the motorised valve. The previous one, installed when the house was built in 1988, failed during my first winter here, nearly 30 years ago. I'll get a replacement actuator and see whether I think I'm capable of changing it.

Actuator is easy, but its the valve that sticks, Scale usually. Work it back and forward with pliers when actuator is removed

My first thought was whether you have a frost thermostat. If the frost thermostat has turned permanently on, or if the wiring from the frost thermostat is shorting (eaten by mice perhaps?) it would cause the boiler to be permanently on.

Before doing that, remove the actuator and see if the valve shaft is tuck. I have (a couple of times) worked it backwards and forwards and it has freed for at least another year! And while the actuator is off, you can fiddle with the controls and see if it moves (although burned contacts won't show up).

Before swapping bits it would be sensible to do a bit of diagnosis... If you have a multimeter, then you can probe the call for heat out of the programmer. If that is on when it should be off, you have a programmer fault. If that works, move to the call for heat out of the room stat. See if that changes with variation of the set temperature. Also check the HW satisfied out of the cylinder stat (like the opposite of call for heat). Then check the output of the motorised valves. If everything up that far seems ok it could be a fault in the boiler's PCB.

There should be enough info a pictures here to diagnose pretty much any CH system:

No frost stat, or mice.

I changed the actuator, and checked that I could move the valve shaft while I was doing it. All seems OK. The boiler's not running permanently and came on and off when I adjusted the thermostat up and down.

Gratz! Another problem FIXED!