speak to these guys, they build proflow exhausts, and they build the exhaust on the car as they go to your specification. heard good things about them and they are based in the midlands.
http://www.ebay.co.uk/itm/PROFLOW-STAIN ... 27b6076f8b
if your down south i would also reccomend hayward and scott, but they cost alot, but the work is good.
as for building a custom exhaust if you go to the right place, they'll do a better job than some of the off the shelf jobs as its a made to measure piece of kit.
like comparing an off the self suit to a tailored suit. but go to a bad tailor, get a bad suit.