It's not that easy to find, that's why!
This is the MoT Testers manual, section 7.3 explains a lot of it :
https://docs.google.com/viewerng/vie...-4-5-and-7.pdf
I'll try and find a more "reader friendly" version of "you don't need a cat if you convert to LPG" but i have seen it online from an official source, i just can't find it at the moment. I've had a long day and no dinner yet but i will find it.