Vaillant boiler lights up every few minutes and heating comes on when it wants. What is wrong?

We have a vaillant boiler which lights up every few minutes, although we do not get hot water (not hot enough to use the cold tap as well, sometimes it isn't even luke warm). We have set the heating to come on twice a day, but it just starts whenever it wants. If we switch the heating off at the boiler, it still comes on. Anyone got any idea what is wrong?