this is a difficult problem to locate without being in front of the boiler. i would start by making sure you have the correct gas pressure at the burner and also check this while other appliances are on such as the hob or cooker, if it is low or fluctuates then get the gas network in your region to replace the guvernor on the meter, low gas pressure is a common cause of constant lock out.
j p ;lewis plumbing and heating fleet, February 2010
