So I bought a 25mm brass metric cup core plug from Core Plugs International (part number BMC25)
https://coreplugs.co.uk/25mm-brass-cup-core-plug-bmc25
It's a little bit deeper than needed, so I ground it down on a bench grinder and finished it off with a file.
I'm quite pleased with the result, but I don't know how it compares with an original part.
Could I have some feedback please from people who own one of these sets and also have the brass cap.
Phil.