My process was:

1. Submit vehicle modification declaring form to Welshpool (This is the stage where they see if you require engineer signatory or not)
2. Since that didn't go through I went to see an engineer to get his signatory
3. Engineer report was then submitted back to Welshpool Department
4. Welshpool department gave me the green lights and to approach a pit to remove yellow off
5. Went to pits, got the yellow off HOWEVER since IM240 was implemented back then I was given a temporary permit in which I have to do the emission testing before I can get permanent permit

So in total, I have an engineer signatory and permit from pits. Cops only look at pits paper coz engineer report means jack shit to them