Good point from Heathy. Some are called 38mm, and others 40mm.
I think Heathfield's main problem is that, in the trade, solvent weld is used. For DIY, push-fit pipe is a different diameter from the solvent weld.
Standard 40mm sink traps are interchangeable. They come with "screw-up" connections that have some tolerance in diameters that they'll accept.
I'm still puzzling over this black piece of tube. I can't think what that is. No matter, easier to replace the trap.