Money missing in available balance?

I went to order a new iphone from o2, I then decided against it and cancelled the checkout (after giving my card details but before I was supposedly charged).

Now my available balance has dropped £10 bellow my account balance. Neither o2 or my bank are any use in finding out why so does anyone know why this has happened?

Added (1). I also have no pending transactions or payments, anything like that

I'm surprised your bank has not been able to tell you why!

It sounds as though your account was "pre-checked" before you completed the transaction, and the "missing" £10 has been reserved. Give it about 3 days and the money will be available again. It's just the way the banks work - it's to try and prevent you from going overdrawn on "pending" transactions.

You are obviously using a debit rather than a credit card. I NEVER use my debit card for anything but an ATM card both for legal liability, and the lock of money you describe.
Essentially, O2 put in a £10 authorization transaction.
An actual charge would correct it to the real number in a few days.
Since you cancelled, the actual charge is zero, and it corrects in a few days.
It is a temporary hold on £10.
If you don't like it, get and use a credit card and then it is a hold on bank money instead of yours.