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Sorry can't help with your wifi issue, but your direct debit issue falls under the Direct Debit Guarantee. This means that BT have to give you a decent amount of notice, usually at least 10 days, of the amount they are going to take. If they failed to give you this notice then they shouldn't have taken the money and you can dispute it with your bank.
A note of caution - check any correspondance you had with them, both paper and online to make sure they didn't give this notice.
Having said that one thing I'm not clear on is how you paid your bills prior to taking out the mobile phone for your son? For example if you received them through the post and paid them by cheque did you stop receiving these paper bills after getting the phone?