Boilermate BP CH on all night despite being timed off with thermostat down

Hi any help would be gratefully received.
We have a Boilermate BP which has not been operating correctly for some time.
During the day the CH seems to respond to the timer and thermostat but at night it takes on a life of its own.
We hear the boilermate switch off in accordance with the timer at 11pm but then throughout the night it switches itself on and off to heat the radiators to a stifling temperature. We have no control over this. We cannot switch the heating off with the control (as it already indicates it is switched off) and we cannot turn the heat down with the thermostat.
We are covered by British Gas but they do not seem to have any idea how this system works. They had insisted the system was displaying no fault until their last visit on Thursday when they changed the motorised valve. This has made no difference and took them 6 hours.
Has anyone else experienced a fault like this or can anyone help diagnose what this fault is? Many thanks

Have you checked the display?
Any fault codes?
How long has this been happening?
Mike

Apparently it is not showing a fault. Although something is clearly not right. On and off this has been going on for over a year. It just seems to be becoming more frequent.

I had nearly the same sometimes. I suspect the valve which switches over heating, cant remember how it is called, it stuck sometimes. When boiler starts to keep temperature, water circulating in radiators because valve is open. But I am not professional plumber so I may be wrong

You are absolutely right. The valve had been changed and that did not seem to help. Then the valve head was changed and this seems to have resolved the problem. The old one was slipping on a few cogs and not closing the valve properly and would explain why the problem did not occur all the time.

Thanks for the feedback, really appreciate. Will check head first when problem comes back again.