Cats hardly ever need replacing and there is nothing in them to 'leak', apart from some condensation. Just try tightening the 'looseness' if do-able, otherwise let well alone. The MoT emissions will tell you if it is working OK.

Hi John

Sorry mate, I think I'm mistaken. It's where the solid part of the exhaust pipe meets the flexibke mesh joint and it's there where it's loose, not the cat. Doh!

Looks like it's a cat back exhaust replacement.

Thanks John.