Had the price been realistic, then the vendor would have been bound by the contract, but 49p must be considered as an obvious mistake and the vendor will not be bound to supply the goods at that price. The law specifically provides for this eventuality. You can only enforce the contract, if you can establish that a reasonable person would have believed the offer be genuine.
Of course, you may still receive the goods, the vendor may elect to complete the contract, or they may simply not realise.
Simon