Normally old half inch copper and new 15mm copper are the same size and can be joined with any 15mm solder or compression fitting.
On rare occassions there is a rogue old half inch copper that is very slightly bigger. I've only come across it twice in 25 years but a good plumbers merchants will stock a compression fitting that will join the two sizes no problem.
Good luck
(boarders You won't catch me with my trousers, Sun 2 Jun 2013, 9:18, 1 reply)

It's definitely just that bit too big, it's the original 60 year-old pipework. I'm using compression joints so it's off to B&Q as soon as it opens at 10.
I'll get one of those joints, didn't know there were such things. It'll be easier than dragging out the old piping to replace it all.

Thank you for replying!
(Juan Quar is clinging to her front teeth on, Sun 2 Jun 2013, 9:51, closed)

it is a fairly rare size of pipe so you might struggle at B&Q for the fitting. You're more likely to have success at a specialist plumbers merchant such as Plumb Centre.
Doesn't help a great deal on a sunday i know but you might have to be patient on this one.
Just recalling, i think the fitting is labelled as 15mm to 16mm.
(boarders You won't catch me with my trousers, Sun 2 Jun 2013, 9:59, closed)

Oh 'ell, we do have a Plumb Centre near here but it'll be closed, and I'm working tomorrow morning!
New question: will a ball joint block off the pipe adequately while I use the washing machine which is fed from lower down the pipe? Or am I looking at a stop end until I can get the special joint?
(Juan Quar is clinging to her front teeth on, Sun 2 Jun 2013, 10:02, closed)

Is the pipe you want to cap off 16mm. If it is, then the only way to cap it without a 16mm fitting would be to flatten the pipe then fold the flattened pipe over on itself. Scary bit now. Apply solder to the flattened pipe to seal it off until you can obtain a fitting.
(boarders You won't catch me with my trousers, Sun 2 Jun 2013, 10:20, closed)

Oooh, that's really living on the edge!

(Juan Quar is clinging to her front teeth on, Sun 2 Jun 2013, 16:59, closed)

Do you mean a ball valve?
If so, it should be fine.
You generally find ball joints on a cars steering.
( social hand grenade I was a lurker before you were a lurker, Sun 2 Jun 2013, 19:50, closed)

I knew that
er...
(Juan Quar is clinging to her front teeth on, Tue 4 Jun 2013, 19:43, closed)