Trying to fit a cloakroom basin ... and find the waste doesn't seem to
fit any standard plastic bottle trap (which would be hidden in the
cupboard beneath)

The threaded part of the waste (a chrome click clack type) seems to
be about 38mm external diameter. but is too narrow for the thread on a
std plastic bottle trap thread to grip.

With it there is also a smooth "extension tube" (10 cm long, non
threaded about 31mm external diameter) which can screwed onto the
bottom of the waste.

Not sure where my inlaw got it from - Generally, it looks to be good
quality... but as I said doesn't seem to fit the std bottle trap...

I wonder if its some sort of continental fitting? And Ideas on the
simplest way to connect it up?

thanks for your help
AJ