Removed the aluminium covers today and found lovely sparkly silver paint underneath. 2 of these had been replaced by the previous owner (different date-stamps on the back) and the newest ones were the ones in the worst condition for ths oxidisation or whatever it is.

Also found that the earlier ones had a ring of glue to hold the aluminium to the plastic, but most of it had already let go. The newer pair didn't have any glue at all.

The centres are now back on the car without the aluminium trims and with some bath sealant to (hopefully) keep them where they belong.