to test for steel - use a fridge ( or other ) magnet . It's copper tho'An update to this: I have my 22mm stopcock and copper metric/imperial olive ready, but have just scraped a patch of paint off the pipe going into the old stopcock and it looks like it's steel*. I tried a couple of compression fittings to 3/4" steel pipe before I pulled it all out and they always wept - is this doomed to failure? I have some water hawk this time instead of tape, which I'm hoping might help.
* There was a bit of tinned copper pipe (?) just above the stopcock, so it might be that.