I know on the A6 c5 induction kits etc are pointless and the stock airbox is actually best. You.might.be better going for an.upgraded panel filter from k&n ect. As heat soak will be your biggest issue with induction kits.
I'd recommend a diagnostic check to see if any fault codes flag up. The knocking in my case on my old a6 is the crankshaft. Hopefully yours won't be that bad.
I'm also in Leicester. The local Audi independent specialist is Chapman's in wigston but I've never used them. I use an independent garage called AC motors in Syston
I'd recommend you speak to autologics or Audi retrofits as members have used them for retrofits and been happy with the results.
I doubt the wiring will be there so you'll need the seat, wiring harness and then it coded to the car.