[David F]

If a consumer orders goods, the trader accepts the order and the price is agreed, but later the trader says the price has gone up, the consumer should only have to pay the price given at the time the order was placed or, alternatively, the consumer could cancel the order. Some contracts where goods are supplied as part of a service, such as double glazing, include clauses stating that the price may be subject to change after a 'survey' has been carried out. If a consumer is not happy with the revised price, the order can usually be cancelled.

From trading standards. My understanding is the price is agreed once payment is taken. By rights the retailer should not take cash till the time of dispatch the fact Tescos and a lot of other retailers take the cash automatically is an error on their part and by rights should hold them open to litigation when these price errors occur