As Martin is one of a small group of owners of older Kent engined 4/4s here I have talked to him about the problem and driven the car a number of times. My guess is that Richard is correct in that something is having a damping effect on the gauge and that could very well be air in the line. Line diameter could also have an effect but seems less likely.
My car, same engine and smiths gauge, reacts differently e.g. 15 psi at idle when hot and max oil pressure 36 psi when hot and running at cruise power. I think this is close to normal.
Just to put the question slightly differently: should Martin continue to pursue the issue of consistently high oil pressure regardless of engine revs? Has anyone experienced a similar issue?
1968 4/4 1600
1950 Auster J5B
Kitfox S5 Outback
1988 Peugeot 205 GTI 1.9