my potterton flamingo 20-30 fires for 30 sec then goes out?

Answer 1

Look at the pilot light. Is it being drawn away from the thermocouple by the draught of the main burner? May need adjustment. Or as other answer suggests the thermocouple may be on the way out. They only last about 5 years.
If you don't know what you are doing get a corgi fitter in; gas mistakes are expensive and lethal!

Answer 2

Normally? this is a thermocouple problem.Easy to get at B&Q and quite easy to fit.hope this helps