[kryogenik]

I think you'll find if you look about there'll be some wording in the terms and conditions saying they have the right to cancel the order for various reasons.
Nothing wrong with giving it a go. They'll refund you if they don't honour it. Nothing to lose?

:edit:

http://direct.tesco.com/help/directt...onditions.aspx

If, by mistake, we have under priced an item, we will not be liable to supply that item to you at the stated price, provided that we notify you before we despatch the item to you. In those circumstances, we will notify the correct price to you so you can decide whether or not you wish to order the item at that price. If you decide not to order the item, we will give you a full refund on any amount already paid for that item in accordance with our refund policy..