what is wrong with my glow worm mark 2 boiler ?

i light the pilot light and thats ok, turn on the thermostat and the timer and the boiler fires up, stays on for a short time,say 10 to 15 mins then stops, shuts the gas off, and the pilot light goes out again, relighting it does the same thing, the radiators in the house are all working fine, the boiler just wont stay on. help, please.