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

The BM will run to heat the store up.
Is the boiler in the garage?
Mike

No, boiler is in the kitchen. BM is on the second floor.
Gledhill has suggested it is a valve fault allowing hot water to flow into the radiators but the valve was changed on Thursday? Thanks

Ops, sorry, I mentioned the same in your duplicated topic.