How to connect stand pipe to McAlpine P-trap?

Should be really simple but has me stumped! But then I'm not a plumber. I have a McAlpine 1 1/2" anti-vac p trap (SC10V) that I am trying to connect a stand pipe to for my washing machine. The connection out to drain is a 1 1/2" multifit threaded end with plastic nut and washers - this is fine - I can get my PVC solvent weld pipe secure in that. The vertical connection is a non threaded '1 1/2" BSP' end with fixed rubber washer and a threaded nut. What size down/stand pipe can connect to this interface? No stand pipes come with a threaded end and my 40mm solvent weld pipe (presumably because it is a 43mm external diameter) doesn't fit. If I get a length of 40mm push fit waste pipe will that fit? I just don't see how the threaded nut would fix that in place?! What will the threads in the nut tighten on to on a bare pipe?