Are all 32mm waste pipe standard size?

No, they aren't. There are two types of 32mm pipes and fitting. Pushfit/compression ones are one size, and solvent weld are another size. You have the wrong type (solvent weld, made of ABS or PVC). You need pushfit, made of polypropylene (PP).

pipe The waste pipe I have is stamped "Marley Extrusions - High Temp - PP - BS5254 - 1.25 / 32" I would have thought this was the right size pipe as it fits my other push fit / compression joints but doesn't fit the outlet in my P Trap as pictured in the link!
I'm totally stumped!!

That seems very odd then. From what you say above, you have 32mm pushfit versions of both fitting and pipe. The pipe should be a firm fit over the slightly tapered fitting you have circled in the photo (then held in place and sealed by a screw collar and two washers (1 rubber, 1 plastic). Is that not the case?
If it helps, I have some fittings like that in front of me now. I make the actual pipe dimensions 34.5mm / 31.5mm (external/internal diameters, measure with a ruler) and the external diameter of the taperd fitting is 29.5mm at the very tip, flaring to about 32mm further back (can't get my ruler to it!)
David

Hi David, Thanks for taking the trouble to take the measurements. Every pipe and fitting I had appeared to be the same diameter as the outlet on the P Trap. However I found a Marley spigot bend I had, would just slip over the outlet and no more. It also appeared to be the same diameter but the thickness of the pipe was slightly less so it just fit. Thankfully I only really needed a small length so just cut the spigot in half.
Still can't understand it though.
Cheers
Stevie