I was thinking, if i get a decat manifold, with a new cat back exhaust system but half way down the center pipe cut it into two and remove a bit to put an aftermarket 200 cell cat convertor would this be road legal and pass an mot?

I would guess as long as you have a cat it wouldn't matter where it was fitted but I could be wrong?

thats what i was thinking the company making the 200 cell cat say with only their cat in the system it will still pass an mot.

just not to sure? its about £550 all together dont want to spend that and then it doesnt work or fails the mot

Shouldn't fail, as long as it looks standard under the car,,,, example,, a lot of people simply remove the cat,,,, cut open along the welds and remove the inner and weld back up again. So in theory, no cat, but it's still there.