In reply to Sonya Mc:
Have you registered on the O2 website? It is quite good. It will report everything about your tariff and bolt-ons, show tariff options, allow topping up, show balance, provide webmail and much more.
ATMs are the often a good option for topping up. Put reminders for monthly top-ups in the phone calendar.
There is a system of 'rewards' for topping up on O2 and these can be channelled back into your balance.
It can sometimes be as hard to get an answer out of O2 as any other uk mobile provider but I do notice that several things are well sorted and predictable on O2. Although it's not as easy to get a good deal on internet stuff as it is on Vodafone, lots of other things are cheaper and one service I use is eleven times more expensive on Vodafone than O2.