Twenty 20: If the buyer has accepted the minor defects after inspection or the seller draws the buyer's attention to them, then I suppose in those narrow terms, sold as seen can apply. But of course, the buyer may bot be able to properly inspect the gearbox or suspension, and yes, some allowances are made for age, price, and mileage. But in general terms minor defects will have to be repaired. It is complicated though and the answers can be found trawling through the case law. I could give you a better answer if a student friend would return my commercial law books to me!:-)
Gouldc: There is no loophole...as such. As stated above, if the fault is specifically drawn to the attention of the buyer, or the buyer has inspected the car, knows the fault is there, but buys it anyway, he has no claim on the seller for that fault. Otherwise, the buyer has a right to reject the car or demand it is repaired at the seller's expense - be it forecourt, trade-in, or even at auction.
I am a keen buyer of cars at auction. Even auctions don't seem to realise (or divulge) that their disclaimers are not worth the paint used to write them - if they are agents for a vehicle being sold by a bona fide dealer. Hence I always use my credit card (so I am covered by the Consumer Credit Act), and buy a car that is clearly trade-in stock being sold by a large main dealer - the stickers on the windscreen give the game away. A buyer has little opportunity to inspect the vehicle in an auction so doing as I do is just about the best advice I will ever give on here.