Bills and accounts / Paying your bill / Direct Debit

1. How are my monthly Direct Debit payments calculated?
2. How can I change or increase my Direct Debit payments?
3. How do I change my bank account details?
4. Can I change the date of my Direct Debit payments?
5. Can I cancel my Direct Debit?
6. How do I set up a Direct Debit?
7. Do I still need to pay my Direct Debit if my account is in credit?