Hi, I had a similar problem with Orange after I cancelled my contract a few years back. If you get no-where speaking to customer services, try asking to be put through to the line manager when you call. If you still get no help ask for the detail of the head office and the complaints procedure, this can sometimes get them to jump to your aid.
If you still get nowhere, contact your local citizens advice or trading standards, and as a last resort contact the financial ombudsman on 0845 080 1800 or 0207 964 1000, all these organisations are there to help.
At the end of the day, yes it is partly your fault for not checking your accounts, but they are not allowed to take your money if a direct debit has been cancelled, and especiall without written notice.
I really hope you get your money back and get the situation settled, and never do business with them again!