Central Heating. GLOW-WORM BOILER?

My Glow-worm boiler runs for a few minutes then cuts out. It heats the water but NOT the central heating system. I have bled all the radiators with the key. HOWEVER, if I run a hose on the main radiator bleed and start draining the system, I DO get heat from some of the Radiators. The system is a gravity feed from loft and NOT a pressurised system. I have run this for several hours over 4 days and then the heat stops. The boiler cuts out after 2 minutes and after a while, they go cold again. Any HELP would be appreciated. Thank you from a COLD household.