Pilot light keeps going out on our Baxi Solo 2 PF?

Hi, we have had no hot water or central heating since Wednesday and would like some help in identifying what to check next. Having called a central heating firm on Friday and explaining the problem they advised we bleed the rads to see if there is any air in the system. We have done this and there was very little air which has meant no change to situation. Problems we are having are:
Pilot light on Baxi Solo 2 PF (Wall mounted Powered Flue Gas type G20,Natural Gas) keeps going out and 'overheat' light is on.
After re-setting system and switching central heating /water on, loud banging noise in pipes and hot water tank occurs- after a few minutes this stops but pilot light has gone out again!
Or if hot water tap is turned on a loud kettling noise and banging occurs (seems to come from the hot water tank in the airing cupboard). We only get tepid warm water and the pilot light goes out!
We have checked the ball cock in the cold water tank situated in the loft and when this is depressed it makes a loud kettling sound?
At a loss as to what to check next, can anyone help?