In article ,
BigWallop wrote:

So look at the black pipe again and gently scrape the outside of it with a
small blade to make absolutely sure that it is what you think it is.

So... Thanks for the replies...

Proper investigation has found the following:

It's plastic and black. Not lead - it bends easy and feels
plasticy. Scraping the surface reveals more black plastic!

Outside diameter is very close to 20.5mm. Wall thickness hard to determine
without cutting it, but it's fairly stiff to try to bend. (but it also
has 8 bar inside it)

It could very probably be dated to the early 60's for initial installation.

So shouldn't be hard to get the right fitting now.

Cheers,

Gordon