I went for a drive out with a friends busa engined car when it decided to overheat. Its been laid up over winter. After 10 miles of trundling at 70 the temp gauge rised quickly and it boiled over. He switched on the fan and left it ticking over and the temp dropped so carried on no probs.for the evening. Now its cooled down despite loosing a god half pint of coolant the header tank is quite full. Do u rekon it was over filled and spat out its expansion volume?!

unlikely to be overfilled, possibly an air-lock developed which overflowed the header tank - might find it releases at some point and the level suddenly drops....also look for the temp gauge moving about quite rapidly which is often a sign of airlocks. I'd start with making sure the system is properly bled by running it up to temp with the cap off the header tank, looking for bubbles coming up - then let it run for 10 mins with the fan coming on/off correctly (or if a manual fan switch put in on at 90 and off at 80).

Sounds like a plan, It's due some fresh coolant so we will have a go

Get it warm ... look for 'any' bubbling in header tank.. rev it..
Bubbles = head and/or gasket replacement

I got to the bottom of this issue, the coolant had turned crusty (in the space of two years) and blocked alot of the smaller tubes,

After some serious flushing and fresh coolant (and checking the water pump was ok) it's happy as larry