The temp gauge was half way up the gauge, so it wasn't over heating...from the gauge reading anyway, it was just warming up in fact. I haven't used the car since last week.
So I pulled over only a minute later as I reached the PO. Popped the bonnet, nothing. No hissing, no steam, no water leaks. Fine.
Anyway one long wait in the PO queue later, I took the coolant cap off and the coolant level was bang on. So must have been water evaporating off the bonnet or something....I hope it was that anyway, as the head gasket has been done only 6 months ago



One down side to owning an older car...I don't trust it!
