It's usually the case, but if in doubt a quick phone call would clear it up.
I've just thought of another reason why this might not work.
Solvent weld 50mm pipe actually has an outer diameter of 56mm.
This means a solvent weld straight coupling will have an internal diameter of 56mm
This fitting is exactly 50mm
So assuming I got it in centrally, there would be a 3mm gap all around. I would have thought that would to be too big a gap for the solvent to work its magic. Would you agree?
I've came across this thread searching for a solution to exactly this bath trap type. I pondered all the same solutions and all the suggestions here just wouldn't work as the parts are just not compatible.
Then I found the real answer. I phone the company that make the trap (Veiga) and talked to them. They told me that what you need is a Mcalpine Multifit-ISO connector. Bit of searching the McApline site uncovered part number Z28L-ISO and it fits nice and snug on the drain and 50mm solvent pipe. You also have the option to cut the 50mm end of the drain and use a 40mm ISO and pipe.
Just joined up to post the answer to this puzzle, because I bet we're not the only two who where challenged by this.