heating clock

Hello, I have a sangamo water heater unit, the clock is not turning. I can hear the clock ticking however it does not turn. There is mains power since when I rotate the clock manually the water heater turns on. Any idea why the clock is not turning? Thanks. Sangamo Quartz 7 Economy

Heating didn't come on for the evening session it had been on in the morning and all fine. At tea time the clock had stopped, no function , no sound. Got new clock and plugged onto wall plate block bracket (have done 15 years before and all was fine) as You'll know it should be that simple. BUT...