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.