Check how much your bank owes you
Firstly, ask your bank to stop paying out any of your money when you go overdrawn. Tell them you'd rather be rejected at the til, or be unable to get money out, than go into your unauthorised overdraft. Set all direct debits to go out on the day you get paid, so there is as small a chance of them bouncing as possible. And ask for an extension of your authorised overdraft if you need to.
This will stop the situation from spiralling any further.
Once you've done thsi, work out what you're owed. To do this, simply look at your bank statements. If you don't have all of them, write to your provider quoting your account number and asking:
Please send me a comprehensive list of all the charges that you have made to my account since 2001, showing the date and amount of each charge, and a description of what it was for. I make this request using my rights under the Data Protection Act and expect your response within 40 days.
You may want to call them a few days later to ensure they've received the letter. Make a note of the name of the person you speak to, as well as the time and date.