I can agree with Andy Coupe on this one. I live in Sheffield, and have had dealings with both Ascar (ExhaustsUk) and Powerfow. The build quality on the Ascar boxes is 2nd to none, and you can get them to build the system to not only look how you want it to, but also to sound how you want it to.
I've had good and bad dealings with Powerflow. It does all depend on the dealer. Where I had my back box fitted on my old car was excellent, but the dealer who fitted by de-cat, was shoddy. The pipe fell off 3 months later, because it wasn't fitted correctly, so I took it back to the other dealer who sorted it no problem.
It pays to check previous work etc before having na exhaust fitted.