Depending on how aggressive you want to be, you could get another MOT station to do an emissions check to test their MOT's legitimacy and possibly report them for a fraudulent one. But regardless of that option, I'd still be chasing them to replace the cat's with known good ones.

16 hours ago, Timeserved said:

Should point out that this garage had my car for 15mths!  They provided me with a valid MOT! After they smashed the cats out!!

To my mind that proves they are con men. I fear your only redress is court action, though it's likely they will vanish as soon as a summons is served.