On Sat, 27 Jan 2018 18:41:06 +0000
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.

This do? https://www.ebay.co.uk/itm/121387297629