Banging Pipes - Hot water only?

We have a worcester boiler (in our new house) which was fitted 9 yrs ago - for some reason it has been left as an open system (Small header tank in loft to feed radiators only!) This may be because we have old fashioned skirting board radiators in two rooms downstairs?? We do get quite a bit of air in the rads which I bleed reguarly. When running the hot water, we can only run it very slowly as if run at a normal or fast speed the pipes bang REALLY loudily (as if the house is falling in) the boiler pilot light goes out and the water runs cold for about 20-30 seconds, the pilot light re-ignites and so the process starts again (about every minute or so). Please can someone advise - could it be limescale, could it be the open system or something very simple?? Being an open system there is no water pressure through the boiler. ANY help gratefully received - Do I need a new boiler??

Nic
January 2012