Potterton Netaheat 16-22 boilier - Water heats OK but rads don't?

I have Potterton Netaheat 16-22 which has banging pipes when the water is heating. This is caused by hot water returning to the feed/expansion tank to such an extent that the tank overflows.
The heating had worked OK up until today when it stopped completely. The timer and room thermostat seems to be working OK. The water still heats and the banging is still there. The pump is working OK (though a little noisy). It has been suggested it could be boiler thermostat?
Any solution are more than welcome.