Decided to have another look at the oil pump in closer detail as had time to spare. Last time I looked at it when dismantling I found what was the source of my bearing failure as there was a small globule of silicone sealant that had been picked up by the pump and lodged alongside the bearing in the non-return valve at the top of the pick up pipe. This "ball" had obviously been circulating with the oil as it had been worn smoothly cylindrical and was roughly 1/16 in diameter.
This time I removed the pick up pipe by driving out the retaining pin in the body of the pump. Inside this there is a 7/16 ball bearing, the bearing on inspection was in a very worn state with rust and pitting. I had another pump which when dismantled had a perfect bearing so I used that as a replacement. I re-seated the ball in the pick up tube with a soft drift and a sharp tap with a small hammer, and then cleaned everything before re-assembly. I assembled the pick up tube to the body of the pump, making sure the bearing was on the right side of the pin - the underneath - and then reassembled the two sets of gears within the pump. It took two attempts before, on tightening the screws, that the pump ran smoothly under fingure pressure in both directions.
As I had time on my hands, and curiosity, I next filled a small film cartridge pot with thin oil and rottating the pump spindle by hand found it pumped up the oil very easily. Then out of more curiosity I wanted to see if the non-return valve worked so I left sitting uprght with a piece of card underneath to watch for drops. 12 hours later the oil wells on the top were still leve with oil and no marks on the card to indicate drips. This was a very thin oil so after 2 hours work I'm happy that the oil pump at least will work well.