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.