Boilermate II Buzzing.

For the last year I've had intermittent buzzing from the boilermateII but the system has worked fine. Until now. Today the buzzing is constant and on inspection seems to be the lower of the two PCB's 92008. The boiler has flShed an error suggesting that there is a problem with the flow thermistor, and I'm thinking it's that the water isn't circulating and it's the PCB's in the boilermate causing the problem. From what I've read though the top PCB is the pump drive and I've no idea what this second PCB's does? The two pumps seem to be spinning although not as fast as I've seen in the past. The green power light at the bottom of the boilermate seems to flicker in time with the buzzing. Any ideas?

The other one is the delay timer PCB.
This keeps the boiler pump running to remove any excess heat from the boiler when finished.
Mike