glowworm swift flow 80 (honeywell) combi boiler?

hiya
my central heating wont work, i've tried bleeding the rads and all seem ok but i think there may be an airlock within the boiler itself. the reason i think this is because when i put the water on i have to put the c/h on as well for it to fire up, after about 15-20 seconds though the pressure guage rises sharply then all you hear is a metallic scraping sound then a clunk then the pilot light goes out. however if i open the hot tap just before it clunks the water jets out as though its under loads of pressure and fills the bathroom in steam then returns to a trickle.
the last time the heating didnt work i gave the boiler a bang and it made a gurgling sound then eveything worked fine.
any help appreciated