Latest , problem solved , after lots of testing and replacements of various sensors , modules etc including ECU we decided to go back to basics despite leakdown and compression tests being ok we removed the head again and swapped with a complete used head . ran for over an hour and no issue ---problem solved , quick removal of the exhaust valves from the original head revealed a problem with valve seats, looks like a different angle was cut on the seats to the valves when we sent the head out for refurb / repair and it had just enough of a seal at cold idle to work and then failed when hot .we had no reason to doubt the work as it is always good , will be doing a full post mortem tomorrow on the head .thanks for the replies guys and