Ok thought I would re-open this thread as I am still having the same fecking problem
Since the original post I have cleaned the external oil cooler up I have also changed engine completley for fresh re-built jobby the only bits I have re-used are the turbo oil feed from the block to the turbo everything else is totally different and not been re-used.
All I can think is that the one way valve on the oil feed to turbo is faulty/ knackered and causing a problem as the Mechanical aftermarket pressure gauge is attached just after this valve.
New filters, new oil have been used, etc...