potterton performa 28i combi?

we drained the sytem to do pipe alterations in bathroom, when finnished we turned all valves back on filling the system, when we turn on the boiler it ignites for about 10 seconds cuts out, the heating works but no hot water, we turn hot tap on, fire up the boiler, there are 5 red lights that come on one after another when it gets to 5 one the boiler makes huge banging noise followed by a big gulping sound of water dropping through the system even though we have released any airlocks via the rads, whilst the gas ignites we've had the hot tap open water becomes hot but then suddenly drops cold due to boiler cutting out, can you advise and save us from this nightmare, cheers. Daz