GB wrote:

Dismantling some ancient piping, I got left with the hot and cold feed
in some old steel pipe. There was some sort of brass connector into that
which was so firm that I could not budge it. Sticking out of that was a
bit of 3/4" copper pipe with one end of a compression fitting, ie the
nut bit and an olive.

Easy! (I wrongly thought.) I'll just attach my 22X15mm compression
fitting to that. Of course, it doesn't fit. The screw thread is different.

What do I need to find to connect up to the old imperial nut?
Preferably, it should be an elbow and reduce to 15mm copper, but quite
frankly I'll settle for whatever fits. The plumbing shop had very little
imperial stuff in stock, and nothing at all that fitted.

I managed to salvage some of the old bits of pipe and it's all capped
off for now, thank goodness.

Is there room to cut the 3/4" copper pipe behind the olive and use a
22mm to 3/4" endfeed adaptor to attach some 22mm copper pipe? I am
sure such adapters still exist and are not enormously expensive.

Alternatively, get the brass fitting off the steel pipe (you can
cautiously saw the brass obliquely to the thread as long as you don't
get to the steel and this will weaken it enough to split it as you turn
it. Open it a bit with a screwdriver (old!) twisted in the cut if
necessary. Then clean up the steel thread and get a very standard
female 3/4" to 22mm compression connector fitting. (If the steel pipe
is not 3/4" then the relevant adapter is probably still available 7/8"
is rare but 1" is common.) The end of the pipe will almost certainly be
a taper thread. In the unlikely event it isn't getting a brass female
taper thread adapter is not impossible. You want at least one of them
to be tapered so you can seal the thread. I'd just use PTFE tap at
3/4" size, but other sealants are possible.
--

Roger Hayter