In reply to Mick Ryan - UKClimbing.com:
Correct post above.
At a supermarket they can ignore their advertised price, as the 'offer of sale' is made at the checkout, and at that point they can choose to correct an error and make a more expensive offer.
Once they've offered and all parties have accepted, however, then they are obliged to supply.
It's not really enforceable though. If they renege, then the only damage you have incurred is the cost of the book, which they will obviously just refund. This means that there's no damage associated with the breach of the contract, and therefore there's not really any liability.
Mark