it is now fitted :D it is a 3 inch reducing to 2.5 inch, came with the hole in correct place and a blanking nut in, removed that and was a straight fit this time round
so now have cone filter, decat down pipe and magnex back box, the sound is great with the decat pipe, had a few loud pops from it already, louder pops than i had before the decat anyways :)