I've not had a chance to get back up yet but I will have another check when I do. I did measure it a bit further back and it seemed quite consistent but I'll measure as close to the floor as I can and see if it's any different.
Sadly it disappears into the floor after a bend. I think the next joint below is an old style compression fitting (swaged type perhaps?)
I had cut off an old 15mm compression isolation valve which it looks like had been fitted with...
Yep, I've tried several different ones, none fit. Not even close. Some just seem to be overpriced 15mm fittings, others are a tenth or two of a mm wider, but none I've tried have been 15.5mm ID.
It's an odd size of pipe, don't think...
Thanks for all the replies, they're much appreciated.
This might be a good option as I think I would be most comfortable with it being soldered or a compression fitting longer-term. But might have to keep some backup options just in...
Well I've managed to run into a bit of a pickle by cutting out some old pipework in the loft and realising nothing I had would fit on to the old 1/2" copper pipe up there. Long story short (happy to add details if anyone's interested)...