I had this once in 1976 - it turned out to be water in the gas supply pipes to the house slowly getting sucked up towards the gas meter - if I listened carefully I could hear the gas pipe gurgling. The gas board came and pumped their pipe dry. They called it, "water on the service."
It will take only a few seconds to check.
Peccavi
January 2009
Check the pilot isnt shrinking when the mainflame is lit.The pilot burners on most modern fires have a small hole where the gas and air mix at the base of the pilot assbly.Make sure that isnt half blocked as this is the usual fault,otherwise i concur that it could be the t/couple
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Try fixfault.com to fix fan heaters.