I would like to thank everyone who advised me over this issue. A senior customer service officer has from O2 has been in touch and have sorted out the issues I raised. They answered all my questions and explained in lay mans terms what had happened. They very kindly and generously helped me with the bill and made arrangements with me where by I changed my tarrif to a more suitable one for my daughters needs and although this will cost me more monthly it will ensure I never get a bill like that again.They also said they will be talking to the call centre to right some of the wrong things they told me. i.e. The connection is secure and can not be hacked by my daughters neighbours, so I can stop glareing at them now !! and that their billing system, although good, can be wrong !! So my advice to anyone else who has this problem is to talk to a senior customer service officer, who will explain it all in plain english and they will help you.
To Sharon, O2,s online manager and the senior customer service officer who rang me yesterday, sorry I cant remember your name, Thank You.I can now sleep at nights and my daughter has been sufficantly appoligetic and with the new taffif in place I should have no further problems.
regards
Andrea