On Sat, 7 Nov 2020 04:51:46 -0800 (PST), Jedzi wrote:

The basic problem is that the very, very old plastic head just turns on
the valve, so either the valve or the head has lost it's thread.

Normally splines rather than screw on, though there will be screw
under a cover to hold the handle on.

I've been to my local plumbers merchant. They told me that you can't
get this type of valve anymore, photo he https://bit.ly/3n2Rdw9

Splines look a bit knackered but a new handle ought to fit and the
possibly bad assumption that they haven't changed the number of
"standard" splines theer are over the years...

I messaged an online plumbing store and they recommended a conversion
kit: https://bit.ly/366v5Kk Unfortunately it came with no instructions
and I can't find anything useful on the web. I've spent ages looking at
it, but can't see how to fit it. https://shorturl.at/hDMX8

That's a composite kit with various collars and washers to enable the
valve body fit 1/2" of 3/4" taps. At aguess you'll need the right
hand collar and the smaller conical washer. You may need to remove
the flat washer to fit the conical one. Not sure what open top hat
things are for. Fiddle about and see where they fit?

--
Cheers
Dave.