Yes "
https://autotopup.o2.co.uk/O2UK/Default.aspx" should be the method but there is no method. CS says they have an 'Issue' but cant say what it is. Fact is some of us have issues with O2 for telling us porkies about their service. Do they listen? Do they fix things? It should be much simpler.
I'd still be interested if anybody HAS found a way of cancelling auto top up on line - Step By Step! Maybe it's deliberately Buried to discourage users...?