There doesn't seem to be any significant noise coming from the pump, and the fault seems to be intermittent.
The heating and hot water had been working fine tuesday to saturday - but the problem re-emerged sat afternoon -i.e. radiators not getting very hot and d2 code appearing. I had inadvertently switched off the power to the boiler in the morning.
Putting the boiler on standby for a few hours seems to rectify the problem.
On sunday eve, the heating was working fine, but couldn't get any hot water.
By this morning (monday), the d2 appeared again - most rads cold, but hot water worked again.
So the situation is not resolved, unfortunately.