Thorn apollo 15/30B boiler?

I have a problem that the boiler runs ok with the burner cover off, but with it on the gas valve turns off after a few secons and if only the piolt is on then this will remain on for a few mintes the again turns off the gas valve. I have changed the thermocouple, linked out the over temp stat, Looking at the electrical wiring diagram i cannot see that it is anything electrical. therefore I am now puzzled. Can anyone please help. the snow is returning and I have no heating.