exactly, baz, the key words are 'legally binding'. But if you bought at auction, or did what auctioneers do, you'd be legally bound straight away. That's because auctioneers take advantage of the Statute of Frauds, section 4, a very old Act, and the old, but still current in modern legislation, Law of Property Act 1925 section 40 which provided for a memorandum of sale. In auctions, that's usually found printed in the auction catalogue. Once that's signed, the parties can't back out without legal consequences. It doesn't matter, short of outright fraud or defect in title, that the property might be itself defective or undesirable or that the vendor might get a better offer.