As far as I understand it, suprisingly goods or services that have been wrongly advertised aren't legally bound to be available at that price. A contract between seller and purchaser is only made when money has been exchanged.
An item on display in my shop has been wrongly priced. Do I have to sell it at the marked price?
A price displayed on the goods is treated in law as an invitation by you to the customer to come in to do a deal. The contract is usually made at the till when the price is agreed. So if you have put the wrong price on an item you are not legally bound to sell at that price. Customers usually only complain when it appears that you are overcharging them!
However as a gesture of goodwill you might want to negotiate with the customer rather than losing them. Trading Standards could also investigate you if they had evidence that you were regularly misleading people about the price you were prepared to charge them. So you should do all that you can to ensure that your pricing information is up to date and correct.
Trading Standards - FAQs - Fair Trading