I initially wanted a decat, but ended up with a sports cat after a wanted ad in the classifieds..

Better I thought, as after previous experience with nackered turbo seals, this may be a happy medium.

So sat in front of me is a universal sports cat, no flange,

I don't want to trash my cat, but that's the only way is it not? I've been quoted £80 to chop my cat to bits and fit the sports cat into the flange etc..

It's 2.75" diameter, that's not 20vt width is it?

Bear in mind that a scap yard will still give you ca. £40 for the old cat with or without flanges

If your sports cat is 2.75", you're going to have to get a flange welded to the front end and a reduced diameter section at the tail end in order to fit it to the remainder of your system