All sorted.
Managed to fit the test gauge (90 degree adapter essential) after removing the plug port for the oil temperature sender which was extremely tight. Good test of the quality of the tool, and don't be tempted to use an L shaped allen wrench!
The oil pressure as shown on the car's gauge was way out, probably only showing 50% at best of the true reading and virtually nil at low tick-over, so that's excellent news. Ok, the gauge I bought was a relatively cheap one on Amazon, but I know which one I believe, particularly as they are simple units and no electronics are involved.
I should add that the engine has only just been rebuilt with new big end and main bearings etc. so I would have been very disappointed if the pressure wasn't excellent. The gauge wasn't showing good oil pressure before but it must have been pretty good in reality.
Out of interest what does the mech gauge show when engine really hot at 4500rpm? Have you looked to see what transducer is fitted as there are varios with different scaling.