Well I am not a lawyer but I am sure someone else will give a legal reply. My take on it would be this. When you ordered the goods a contract was formed. They agreed to supply the goods and you agreed to pay the price. However you did not agree to purchase a better item at a better price. If you returned the goods and asked for a full refund plus the thirty pounds then I think that would stand. The trouble is with on line shops, do you know who they are or where they are ? If so could you call them and ask to speak to customer complaints or something.