I have been charged 6.42 and I have not purchased anything

You've checked the purchase history on your account (you didn't have any pre-orders or auto-renewing subscriptions that were due ?) : http://support.apple.com/kb/HT2727

If you don't recognise the charge, and nobody else has access to your account or devices and may have made it, then you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption