On Sat, 10 Oct 2020 01:10:03 -0700 (PDT), Jedzi wrote:

I've had a heating engineer round and he thinks it's the pump or the
boiler thermostat. He said that the pump looks & sounds like it's
working, but he can't be sure.

Assuming a normal type CH pump can you get at the slotted cap in the
center of the motor housing? If so remove it (there will be a bit of
leakage) and insert a flat bladed screwdriver and turn the rotor
easily manually and check it's spinning fast when powered.

Another possibility on a system that old is build up of sludge
somewhere that is restricting the flow. Maybe in the heat exchanger.

--
Cheers
Dave.