You only need go through the complete full CAT test procedure and choose vehicle from database if it fails the initial BET test.

I know the rules were whatever is oldest car or engine you go by that, as I didn't need a cat on my mk2 Golf (1991) with a 1995 VR6 engine. I think, as Gingerturbo says they've changed the rules, I've been out of testing 6 months so not up on the latest changes.