I've just been on the phone to the bank and they said that they'll send me out a bunch of forms to investigate the transactions. The O2 phone ones are on a fraud one and the other two will be on a disputed transaction form. Basically a lot more hassle if you've got a debit card as it's actually your money that has gone out and they need to investigate it before they'll refund you.

I'm just glad that they hadn't tried to totally clear me out in one go...