Help ID-ing a fine pitch thread, OD ~23.5mm

Hi folks,

I have finicky leaking isolator valves in a Hudson Reed concealed shower valve assembly that I'd like to replace with simple plugs in the hopes I won't have to tear into this again. Trouble is I can't find any info on what thread size this might be.

It's a bit bigger than a 1/2" BSP plug I had kicking around, and the threads are much more finely pitched. Looks like twice as fine or more than the BSP thread.

Hudson Reed is a UK company I believe, is there a thread that makes sense that they would have used here?

Pics attached. Many thanks in advance!

Remove the "O" ring and solder over the flat end refit the "O" ring and you have a plug.

I like the suggestion Jim, thank you. Trouble is no soldering stuff where I am right now, and also these things are leaking both from the tiny isolator screw and o-ring in their centres and from the o-ring and threads around the outside of the fittings.

I'm stuck having to order something to replace these due to the lockdown, and I don't want to just buy the same thing from Hudson Reed and have it fail again. So really need to figure out the threads. Looks like something like 24tpi but am at a loss here on the spec. Cheers.