Big man from the west coast bagged himself a rare beauty and worth every penny in my opinion, oouur wriiight ... ... Sir!

I would say standard S6's with Climate and leather with no rust with average miles and a good history are worth £3k and more depending on what options it might have. Goodies from the factory Options list make any S6 more desirable and sought after. Electric seats,Cruise,
Skooshers 
etc adds to it's value.
Problem with pretty much all these cars that turn up is they are
very tired in the suspension department necessitating immediate work which is not cheap and the reason why many owners pass them on.Its very rare to see an S6 with receipts or documented evidence of full suspension/ steering rebuilds that cost minimum of £2k plus for parts alone! So A car that drives and rides like a new one would easily be worth £5.5k, in my opinion atm.Macboings Stunner with 50k miles would be worth more and the price it sold at a year or so ago was cheap! It's shocking how prices have gone up during the last 2 years, even just the last 12 months.
They are brilliant cars
if they are in very fine fettle!
