so for anyone who picks this thread up I have some conclusions.
If you get a bike which has been stood for a very long period of time, in this case 25 years, and the oil level has steadily dropped (when I purchased this bike it was just below the minimum, so below the strainer) the chances are you will have to prime the oil pump.
I have removed the sump and investigated and the oil pump is bone dry. Now as there is no debris, wear metals or sign of distress in the bottom end I will assume the pump has been turning when cranking. As such if one of these pumps does dry out it will not self prime (makes sense)
Solutions - in my case I have removed the sump, but i think having looked at the design of the oil circuit in the sump, I could have gotten away with pumping oil in though the oil cooler inlet pipe so outlet from the sump (right hand side of the sump when sat on the bike). In my case I will pump a heavy weight oil (such as gear oil) into the pump (somehow ) and then reassemble .
I will report if I have success.