Deb,
As long as you don't get an exhaust with twin DTM tailpipes then you'll get away with arguing the toss over the following points.
- It's not power increasing (how can they prove otherwise?)
- You're buying it because your one fell off, and Renault want to charge you £800 for a new one, whereas this will cost £300 with a warranty.
- You'll happily take your custom elsewhere if they try and charge you for something that doesn't change the car (other than sound).
With that they might still try and charge you something (which you will still say no to) and eventually you'll get what you want.
At the end of the day, whether you have a side exit straight down pipe or a halfords backbox, whether you pay £1 or £750 extra insurance for an exhaust, it's still marked on your policy as a "modified exhaust system"