I dawned on me yesterday as to why these W2's have this problem. I reckon the rocker stands were delivered per the standard configuration and have been subsequently milled down flat for the W2 rocker stands (aluminium blocks). This would have effectively removed the area for oil to reach the stands in question.

Anyway, he has now oversized the hole around the base of the bolt and widened out the area where the oil gallery comes up through the head. Hopefully this will provide sufficient room for oil to pass through the head into the rocker shafts.

Will fire it up later today and report back. Still have to do the other side, assuming it has the same issue.