Any thoughts on this, no airlock, motor not over heating,it did sit for about a month, when starting yesterday, noticed fan came on straightaway. i thought if the thermostats crook it usually stopped the fan working, is this a foible of gasser? 

Thanks in advance.

Sounds like the the thermo switch is stuck closed inside.Might try some very gentle ''tapping'' on it and see if it opens??

Do a continuity test on the switch terminals with the engine cold/off. 

thanks Team, she's US. have replaced is there a common thermostat that applies to other uses / makes that would suit or does it have to be gasgas supplied ?

Cheers.