Play.com don't have to honour the price. Here are points 4 and 5 of their Terms and Conditions:
While we try and ensure that all prices on our website are
accurate, errors may occur. If we discover an error in the price of
goods you have ordered we will inform you as soon as possible and give
you the option of reconfirming your order at the correct price or
cancelling it. If we are unable to contact you we will treat the order
as cancelled. If you cancel and you have already paid for the goods,
you will receive a full refund.
No contract for the sale of any
product will subsist between you and Play.com until Play.com dispatches
the product(s) ordered. We will confirm that the product(s) have been
dispatched by a confirmation email. This confirmation e-mail amounts to
an acceptance by Play.com of your offer to buy goods from Play.com or a
third party supplier that is engaged on your behalf by Play.com
(whether or not you receive that e-mail).