Potterton Netaheat bolier main burner lights but flames low?

My Potterton Netheat 10/16 eletronic boiler lights up OK - piezo - pilot - main burner. But I get no hot water out of it (outlet pipe stays cold) and the main burner flames are small. i've replaced the boiler thermostat but that made no difference. This came on suddenly when we noticed no CH / HW. Any advice would be greatly appreciated, TIA. Terry.