Might be bad form to reply to my own post but here goes.. I figured the coated end needed to go in the cat side. No sealant.. All buttoned up now and not blowing but it was difficult getting the thing to seat properly, needed a wire brush in there and a little opening up with pliers on the tangs on the end of the pipe. It's also impossible to visually check it's seated all the way round but I used a tiny flat screwdriver to "feel" the seating at the end of the tang slots that can't be seen.
All good, another little job completed. Brake pads next.