A number of online retailers (such as iTunes and Zaavi) run test transactions to check whether a card is valid, usually for £1. This is either done when registering the card to an account, at the time of purchase, or at a random interval in between.
The funds are temporarily blocked for around 10 days, after which they're released back into your balance. If there is another charge that you do not recognise, please contact us urgently via our customer service team on 01962 676080.