A merchant must state the total amount to you before processing your card details.

In some retail outlets you even get given the card reader to check the amount and press "OK" if you are happy with the amount before you put your pin in.

Seems a little deception has took place.

Go to your local magistrates court and send a small claims letter that you fill in yourself explaiining that if you dont have the funds that were unclear refunded, that you will proceed with legal action.

with the refund of these monies then you are prepared to overlook this error and not take it any further.