Does a shop have to honour the price it shows on the shelf?

No, if the displayed price is a mistake. The price tag is not a contract. It is an "invitation to treat" ie it is inviting the customer to make an offer to purchase and the retailer doesn't have to accept that offer. A contract is only formed when the shop accepts a payment, after which it can't oblige you to repay any difference between the real and the advertised price. It's more complicated if it's an online transaction. Deliberately misleading or inaccurate information is punishable under law. If a merchant refuses to honour an agreed price – say a builder's bill is unexpectedly and significantly higher than the quote you accepted – you can settle the bill, but note in writing in a letter and on the cheque or credit card slip that you are paying under protest. You can then seek to recover the balance via the Small Claims Court .