Does anyone on here have an idea what may be going on with said heater when the hot water suddenly turns to cold water please as I find it baffling ?

Thank you.

If the radiators are also coming on, my bet would be a diverter valve problem

Have you checked whether the heater is genuinely cutting-out or whether the heater is working OK and the problem is elsewhere?

It had this problem the other week and a council plumber came out and tried to see if he could get it to cut out but couldn't. The radiators came on fine though when he set it off.