hammering noise

hi there,
i'm having problems with my glowworm fuel saver 35f gas boiler. When you turn either the hw or the ch on the boiler will light for about 30 seconds and then start banging loudly and cut out. sometimes the thermo cut switch on the bottom will trip but often dosent. i spoke to one plumber and he said it sounds like the pump, to which i have replaced with a wilo gold 15/50. bled the system. no good

i've tried bleeding the system and the rads, it will discharge air but seems to run out of pressure before any water is released from the bleed valves. also seems to have an awfull lot of air in the system even before i replaced the pump.

i live in a flat so have no tanks in the loft etc but do have a hot water cyl with a little reservoir bit on the top.

any help appriciated, thanks

Sounds like you have either a sealeds system which needs topping up or you have a blocked cold feed. id try and locate yout central heating header tank. Is the tank on top of the cylinder spilt internally into 2?