Hopefully found the fault, looks like its the header tank cap, I have the overflow into a small bottle with a little coolant in , as soon as the slightest pressure starts to build it escapes into the bottle , had a new 13 lbs cap so changed it for the original - no more bubbles in the bottle , took the car out for a decent drive today, heater worked fine all the time, temp stayed at 1/2, as soon as I pulled up at home I popped the bonnet, the coolant was bang on level no bubbles in the bottle have ordered a correct 16 lbs cap + I will flush and refill with the correct coolant Mine has the aux heater so I will remove the pipe under the car to drain just to make sure I get all the old coolant out Thanks Dave