My Boiler Pilot Light Goes Out When The Central Heating Is On?

I have a Glow Worm Economy Boiler. The pilot light goes out when the central heating is in use but stays lit if I only heat hot water. I know the thermocouple is ok as it can run heating the hot water for months no problem but the pilot light goes out every 30 mins or so as soon as I put the cenral heating on.

I can't understand what can cause this any ideas?