I would check out what you can get for your money at £370.00 as I got two separate back pipes with boxes made out of stainless steel handmade and fitted for £300.00
you can see them in my build thread the "yellow project"
cheers
stick to your guns mate, you are right Nissan across the UK have got a bad name for not checking their performance cars and if you are like me I know right away when something is wrong with my car as I become part of my car when I drive it!
I would not say £114.00 is a rip-off, I had my car laser aligned and was taken around the car as they were doing it and I have seen for myself the work that went into the car, I paid £109.00 inc Vat.
To me Nissan is out of line here, pardon the pun, it is a high-performance car, and all this should have been checked for you as it affects the driving of the car.
try citizens advice bureau for legal help. Best of luck mate