You know what? it may have just been some water not from the head gasket, possibly from the time standing or the rebuild.
I was thinking head gasket as i had seen some coolant on the rear cylinder just under the exhaust port. But the weekend with the pannels off after a spirited ride i noted the coolant needed topping up and with the left panel off the water pump was covered in coolant.
So ran it up to temp and could see a leak just behind the pipe.
So coolant was running back over the pipes and onto the rear cylinder!!
So stripped cleaned and sealent later we shall see.