Solvent weld pipes difference between metric and imperial ?

This is/was problem with plastics in plumbing - there is no standard size, each manufacturer made there own make slightly different in size. I suppose it was an attempt to corner the market, in that if you bought say Quicksnap pipe then you had to use quicksnap fittings.
A few years down the road Quicksnap gets taken over and discontinues making Quicksnap pipe and fittings.
Hence, there's miles of the stuff been fitted, but you can no longer get the right fittings for it.

You just have to use your initiative when connecting onto it - you get buy a universal coupler - but it's quite an large ugly compression fitting to join up with and is not ideal.

Actually 1mm not a lot to make up so just put plenty of solvent on the joint.

The manufacturers used to make a gap filling cement which was quite good, however, I've not seen any for years now.