Was it a pattern cat ?, I would start by cleaning the maf, using carb cleaner clean the hot wires inside it, don't poke it with anything it's not needed, then pull fuse no2 a violet fuse marked KAM, this will reset the ecu to learning mode for best running/settings.
Then you need to give it a seriously good thrash, surprised the mot centre didn't tell you this, very very very common this is, esp with pattern cats that only just scrape through.
My dads just mot'd his mk1 2L ghia , and had to put it into 1st(auto) and do a mile with it hitting the limiter after failure, but easily went through on return, good job I built the engine right!!
tabetha