So you've got a full cat-back Milltek then? If so it should come with a 2.5" - 2.25" reducer to join to the cat.
The old system is a single pipe from the cat so you've got 2 options, first is to drop the rear beam to remove it, second is to cut it close to the rear beam and remove it in 2 sections. I went for the latter when I fitted mine.