This is jsut my opinion but I used to have a 1.4 clio and had 4 different systems at one point or another.
Firstly dont expect a power gain from a backbox. So long as you dont then thats fine. It will look and sound better and you will probably drive your car harder as a result, but you wont gain power really.
Due to this, I dont think it is as important to go with a proven company that conduct R+D into their products. If you were going for a full system this might be a consideration (it would be for me) but with just a backbox it wont really matter.
So what the decision usually comes down to is tailpipe design and how loud you want it. On both these fronts the custom exhaust places like Powerflow, Top Gear etc are your best bet, but be aware that as they are basically franchises, the qualtity can differ considerably between dealers.
Personally I can recommend Big City Tyres in Nottingham for weld quality and service but dont mind paying a bit more, or Top Gear in West Brom if you want it cheap and dont mind waiting around whilst a pre-pubescent child fabricates your exhaust.