Jeez!!! If bergs had a pressure gauge it would be off the scale by now......
Worty. That pesky valve is in the top hole nearest the timing bush. It consists of a ball bearing pushed under light spring pressure onto a crude seating,( as detailed by Rob,) towards the pump. It cannot be accesses from outside, only by splitting the cases. Later A65 design overcomes this, and is an A10 modification detailed on the Forum. With the pump removed the ball can be pushed off its seat with a matchstick or similar, and be felt to return.
For those folks in a similar situation, crude way of cleaning the oilway and seat is to remove the pump, hold the valve open and apply aerosol solvent cleaner, eg carb, brake ,WD to the centre hole where the PRV Mounts. This should backflow any debris in the oilway. Follow with a a good dose of compressed air, valve still held open, finally pump oil down the oilway, observe exit from the PRV hole. Pumping oil the other way from the PRV should result in no flow from that pump oilway if the ball and seat are good. A weak, broken or jammed spring means the valve won't seal well enough to prevent draindown.
I'd stress this is not ideal, but can go some way to improving the sealing of the ball without major dismantling, if the problem is muck and sediment sludge on the seat and ball. Simply pushing solvent down the oilway washes debris to the timing bush, BAD, at least this method keeps trash out of the motor.
Those cases look simply stunning for attention to detail, so I doubt simply cleanliness is a factor. Ball in cam feed still an enigma. Fair play to MWAS and bergs for bringing it back from the brink. Give it a present, punt the switch, sleep easy.
Swarfy.