Well, the head gasket has gone.
I noticed the temperature gauge going up on the way home today, and fortunatley it was just under 3/4 when I got home.
While idling the engine was putting small amount of white smoke from exhaust, some bubbles in expansion bottle, and it had lost some water.
A local mechanic said he would change the gasket, skim the head, re-seat the valves and give the engine a general check-over for Ã‚Â£150, which I didn't think was bad.
I'm not going to drive it until its done, as I don't want to damage the engine.
Are there any special precautions to take after the gasket has been changed, ie. to run it in?