they're not THAT cheap. My garage recommends Bosal for a cheap OEM style replacement, I think they run to about 200 quid. I got my custom stainless Powerflow Cat Back system for about 240 so Powerflow got my business. They did a good job although I have heard horror stories; much depends on the workshop, from what I have heard.
Sportex also do a mild steel cat-back for about 170 quid; I tried one out and had to get rid of it straight away; it was much, much too loud, despite the advert promising a 'non-intrusive' engine note, much happier with the Powerflow - although stick with 3" or smaller tip otherwise the bumper melts!