Centre section and back box, fitted through Renault, £380 sounds about right.
If you're going to get both done, you could get a performance stainless steel Cat back system (ie centre and back pipes) for about £300.
On my old Saxo VTS I had a Powerflow cat back system, stainless steel, lifetime guarantee for £299 fitted. I don't like loud exhausts and asked for it to be as quiet as possible. Was louder than standard but nice enough.
The only reason I went for a standard back box on the 172 was that the centre pipe was recently replaced under warranty and the back box needed doing asap and Renault got me in the next day.
Plus, I like the exhaust tucked out of the way like it's meant to be on a 172. You can get stealth systems to replicate this but I was skint and it costs more.
Please whatever you do, don't take it to some kwik-fit style place.
The exhaust is a sensitive part on a performance car and allows the car to exhale and inhale properly. A crappy system will hamper this, sapping power.