Stuart
Had a similar problem with a friends Vx powered 7, the coolant system was pressurizing with a build up of air at the top of the rad. We got a restrictor from Caterham that fits in the small hose to the top of the expansion tank. It stops the coolant from airating in the expansion tank. Haven't had a problem since.
It could be your head gasket but I would explore the cheaper/easier changes first.
Nick
Mellow Yellow Vauxhall - I don't think so!!! π¬ π¬