I am a manager of a plumbers merchants. We deal with both installers, diy'ers and retail on a daily basis with the same problems non compatible fittings. Some of the traps will take a McAlpine Eu to UK adaptor rubber to the outlet on the trap.
We used to be a couple of hundred yards from a Bath store and were there unofficial customer service counter.
We get people in who get the hump with us because we don't stock the parts to fit parts bought via the Internet. Most of the products are a pile of crap.