About 10mins ago, my central heating control unit - my heating was on at the time - starting making a vigorous clicking noise. Went to investigate, tried switching to off position, unit not responding just continues to make clicking noise. Heating still on, pilot light on etc. I just can't switch the unit off. I switched it off at the spur and the unit and boiler went dead. Waited a few minutes and switched it back on...the clicking noise is still there. Any ideas on diagnosis.