Update guys -
We went out again tonight and the following -
Temperature gauge went funny so we pulled into a nearby carpark. Coolant tank was empty and pressure was getting past the jubilee clip on the thermostat housing (a vast amount of air coming out).
Waited for her to cool down and filled it up again (at least two litres of water went in)... started it up with the cap off and the water started to expand out of the expansion tank.
A friend and I are positive its head gasket but the owner doesn't think so.
Opinions please?