Problem sorted.
Just found that the screw on the block next to the thermocouple connecter is used for calibrating the thermocouple.
Noticed that the vibration from the starter motor is just enough for the little blighter[:@] to turn itself backwards slightly.
Turned it forwards about 10 degrees and all works fine, will reset the value for start up manually, then a dab of paint on the screws head & sorted.

Thank you all for your help,
Regards,
John