We have a Vaillant Combi about 7-8 years old. It has been playing up with the heating not coming on with the timer. Hot water is leaking slowly out of the overflow. The hot water alsways works but now the heating will not come on. The boiler is the right pressure, I have tried draining and re-topping up. Re-lit the pilot and it keeps trying to fire up the heating then stops again. If I turn the main heating switch off then on again the pump is constantly running even with the heating off and no hot water triggered. Any ideas as its rather cold?