Heating fails when overnight temp drops below zero

I have recently moved into a house where the heating is through a Boilermate OV system. Recently with temperature dropping below zero at night the heating does not come on in the morning whenever there is a drop in temperature. On normal days it is the heating works ok. The hot water supply is usually fine although occasionally that also plays up. The boiler is in based in the kitchen so inside the house. We had an engineer over and he changed the rocker switch but the problem persists. Trying to avoid calling the engineer back, he was clueless. Would appreciate any suggestion. Many thanks

It's happened again this morning. The heating did not come on in the morning. The hot water was fine. Would appreciate some suggestions please. Many thanks