Hi Plotter,
I understand. You are right it is Section 75 that catches the lender.
To try and put this to bed -
As a buyer you're best friend if it comes to a legal case, or threat of one, is the Sale of Goods Act which gives you some rights and the seller some obligations, the usual relevant bits being that the goods are correctly represented (so a 10 year old caravan is not sold as a 5 year old, or it it represented to be damp-free when it is sodden) and that the goods are fit for purpose, free from faults, and of satisfactory quality.
There's a a quick summary here from the DTI:
http://www.dti.gov.uk/consumers/fact-sheets/page24700.html
Section 75 means that the lender, in the circs we are talking about here, is just as much on the hook legally for this lot as the seller, and from a legal point of view that is very valuable as they are usually banks who (a) have lots of money (b) care about their reputation, and (c) would usually rather settle than lose in court if things look serious.
Be prepared to push it though.