I have found the leak. I removed the radiator grill and rad supports. Heated
the engine up real hot by disengaging the fans and running the engine at 3k
revs. Once the temperature gauge reached 3/4 way up (Fans kick in 1/2 way
up), i reduced the revs and left ticking over. The radiator has a 1"
hairline crack in the top plastic, hardy visible under the grime. When
cleaned, the crack spurted coolant a good 2 feet in the air under the
pressure. Sorted.
To fix it i cooled the engine, with a soldering iron, melted the plastic to
seal the crack, anc covered in Epoxy. All seems fine now. Will report back
in a few days if no more coolant is lost during driving.