I am sorry postdog, but your answer is incorrect. In English Law (Sale of Goods Acts, common law, case law, precedent, Tort) are very clear, once money has exchanged hands, a contract is formed and the Seller must honour the contract. Three steps: (1) Invitation to treat - when the item is advertised/displayed for a price (2) Offer - you make an offer (in practice you pay the price displayed (3) Acceptance of Offer - when the money is taken from you.
If Play have taken your money (Credit/Debit Card), then contact your local Trading Standards Office. They have all the relevant legislation, regulations, case law at hand for this kind of thing. Play cannot claim a 'mistake' and get out of it if they have taken the money. Until they take the money they can cancel an order at any time. For example if you walk into a shop and see something on the shelf for �200. If you then take it to the counter with the �200 and the counter staff decides they don't want to sell to you, they can do that. However, if they take the money (physical exchange of funds, whether they give a receipt at this point is irrelevant) and it is understood that they have accepted this money for the item, a contract has been formed. If they than realise that it was mis-priced by say �500 and should really be �700, not �200, they cannot do anything about it.
The law is on your side use it. However, if they did not take money from your account, then sadly they win. It doesn't matter if they have refunded the money. The contract for a printer @ �54.99 has been formed, they have to supply. Refunding does not absolve them of their legal responsibility unless you have asked to be refunded.