Hi Phil
Referring to your top photo, the central disc arrowed in yellow should have a 1.5mm hole in it approx 10mm from centre and 20mm from the edge. It looks like a new centre on yours held in place by the R clip. Mine is original held in place by a star lock washer. The hole is there as a breather hole. A 1.5mm hole drilled anywhere on the inner raised part would suffice.
Keith