Well I thought I'd update this for anyone in the same boat in the future.
The plate that joins the Cat to the manifold seen here:
Mine however did not look like this and had rusted away so rather than two holes I just had two half circles. How the hell this manged to happen I don't know as the rest of the metal looked alright. I wasn't keen on shelling out some money on another cat when mine works perfectly fine so I figured I'd weld a new plate to the end. I initially planned to remake the whole thing as one new piece as I didn't think it would weld very well, However it actually welded pretty well so I welded on a couple of nuts on instead.
I fucked off the original bolts and replaced them with some threaded bar, a few washers, original springs and double nutted the ends. All in all it seems pretty solid, Slight leak when its cold but some gun gum should sort that out.