Boiler firing up when not on cycle?

I have what looks like a fairly old-fashioned heating system. There is a gas back boiler behind the fire in the living room and an airing cupboard storing hot water. Next to the fire there is a switch, a light and a slider that goes from high to low.

The problem is this: it seems to have a mind of its own and I would bet we are wasting a lot of gas. We had the heating and hot water set to come on between 4 and 5, morning and evening, but the back boiler appears to fire up (the light turns on and it makes various noises) at random times throughout the day. The airing cupboard seems very warm and it appears we are wasting a lot of heat. Is this normal? I would have thought the boiler should only come on when I tell it to.

I suppose the rather warm airing cupboard is due to lack of insulation because the system is so old, but it seems like it is heating water nearly constantly.

Any advice would be much appreciated. Thanks.