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When the phone sleeps, wifi is switched off and reverts to mobile/cellular data thus any apps updating in the background will use data. This is almost certainly the trigger for the £1 daily web charge. Switching it off will stop the charges.
You can get an itemised bill but you have to fill in subject access form and enclose a payment of £10.
Other possible charges you are incurring could be sending mms instead of sms. Explained here :
Accidental MMS Sending and How to avoid it by MI5 - Here
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