I wouldn't bother if I was you tbh. Just cut out the muffler in the middle of the exhaust, weld a straight pipe onto it and then take your old backbox, cut the exhaust pipe as close to the backbox as you can so you keep as much of the pipe as possible and then attach any muffler that takes your fancy 
Greggson make 2 or 3 different exhausts for the Micra and I would recommend them, but I simply did what I described above and then bought a performance 3" muffler similar to those you might find on a Ford Focus and bolted it up and it fits fine, sounds great and has made the car accelerate better not much better and not like better, but still better than it was stock
