What size exhaust flexy, and decat?

[one-eyed riley]

Hi folks
I have the usual leaking exhaust flexible pipe bit and need to get a new one welded in - to save me buying the wrong one off ebay, does anyone know what size (diameter) to buy please?
Also I am considering getting a decat pipe fitted at the same time (2003 ZT diesel). I have scanned the many posts on this which seem equally divided between those who say it doesn't affect the MOT and those who say it will fail it. I presume people have decatted without penalty. Has anyone actually been failed because of the lack of of a cat please?
Thanks