I would reccomend Longlife Cardiff, I purchased a Powerflow from Topgear Bridport years ago for my 206 XS and it sounded horrible and did not fit correctly and much slower acceleration. To fit the extra tailpipe they cut the rear bumper with a hacksaw and did not even sand it smooth it had stray pieces of plastic hanging off. After complaining they took it back in and still did not fit properly so they sent me to Longlife and told them to sort it which they did they found the diamater pipe they had used was for the 1.6 16v (which I reminded the fitter twice there were two different 1.6's) instead and they reduced it by a quarter of an inch and the car sounded much better, fitted correctly and performed like it used to which to me was great!
Since then I and friends have used Longlife and none of us have been dissapointed. Thumbs up I say!