I had a Powerflow system fitted in December, because I couldn't get to Tony Banks in Leeds.
£240 Backbox only or £400 for a full cat back system.
Last month my brother had a full cat back system fitted by Tony Banks to his 75 CDTi, lovely job, but I can't remember how much he paid.
Both systems were specified to be as quiet as possible, neither system is as quiet as a standard Rover system, but neither are they intrusive. Listening to my wife boot the ZT-T out of our street, it has a nice restrained burble to it, but is still perfectly quiet at 70mph on the motorway.