I think the jist of the original post may have been lost.
If it is not fit, why has the owners carried on using the vehicle? (see reference to 600 mile trip)
To reject it, the owner should have returned it within a reasonable time (three or four weeks) and discontinued using it.
Yet they have done neither of these things, even after advise from a breakdown company and after another garage has surmised there might be an issue, which is why I am expressing my view they are to try and resolve this in a different way.
To go direct to court to claim for a fault:
No one quite knows what or why or when it developed.
No one has confirmed it actually exsists.
That the seller hadn't the chance to inspect, put right, exchange or refund.
And the owner has carried on using.
And expected to win? More like loose a lot of money.
By all means go to court, but please have confirmation there is a fault, it's for you to "prove" these probable, not the dealer to prove they weren't.
That it was extremely probable it was there when you bought it, so the vehicle was not of satisfactory quaility at that time.
Or did not meet the description given.
When considering the above, it might be worth checking out the Audi TFSI situation as oil consumption can be a very contentious area.
Audi fully admit there is an issue with piston rings and oil consumption on certain engines due to manufacturing, but will only review each car that is out of warranty, case by case, with no promises of footing the bill for repair or replacement.
Amazingly, they are operating within the same laws as everyone else!
If the car was not described as being in excellent condition or good working order (often these sales are referred to as Sold as Seen, Trade or a Part Exchange vehicle), your rights are very limited, if none exsistent.
If you are in any doubt you cannot fulfill the above, I stand by my original post of trying to sort this out with the dealer amicable.