@jenhenry - welcome to Talk3G.

Thats very interesting - it sounds like O2 have somehow not accounted for your usage. Have you registered for an account on the O2 portal so you can see on-line what your PAYG usage and remaining allowance is? Perhaps a rich uncle has donated some huge top-up LOL

I don't think O2 will charge you retrospectively the next time you top up. There are no words in their Ts&Cs to say that they'll let you run free until the next top up. Besides which, such practise would be seen as liable to induce excessive usage and thereby charges to the customer that would be unreasonable.