The fault began about after 100 miles of motorway driving when the temperature guage started to flick from mid position to hot and back again . On opening the bonnet It became clear that the cooling fluid had been blown out of the expansion tank . I refilled this and tested the car for about 20 miles the next day on local roads and everything seemed fine . On the return motorway journey the same problem occured after about 110 miles. I have since been driving too work locally about 4 miles in traffic ( 30 mins ) and although the level in the expansion tank has risen about an inch I have had no further problems . There are no visible water leaks and the car does not seem to be losing water.Is this likely to be a head gasket problem ?
Seems the cooling system is pressurising when engine put under load , suggesting head gasket leak . Easiest way to check is to get a sniff test ( block test ) carried out and eliminate that possible cause . A pressure test on cooling system would should any exterior leaks i.e water pump etc . I would start off with these checks first before getting too involved with other possible causes.