Thanks for the tip Ed.
I have recently spoken to Alan Gibbins and I now have a better understanding of the rocker oil feed situation and he enlightened me on the differences between the original and later design rocker shafts.
When I first inspected the rocker bushes I was convinced they were a single bush with a central circumferential groove but I now realise this was the gap between two seperate bushes.
On closer inspection of the shaft, I found that the 0.003" flats were hardly visible, due to slight wear so I decided to caerfully reinstate them with a miniture diamond grinding disc. The net result was an improved oil flow from all the pushrod and valve pad drill holes.
John.