they owe us more than we owe them!

mumofflo

My husband ran up a credit card debt with Halifax before we met. I helped him to start paying it off. He also has an overdraft with the same bank and this went over the agreed limit when they took an unauthorised payment for the credit card which we had already paid that month. This incurred charges on the account which made it unmanageable. We have since requested charge details for both accounts and found that the charges and interest that he could reclaim are DOUBLE the outstanding amount on both accounts. We have written to them letting them know this but he is still getting in excess of 10 phonecalls per day from them and if we do answer they just demand payment and cannot deal with any enquiry about charges. They have now sent a letter saying they will be visiting the house. A few questions:
Bank cahrges reclaiming is on hold but what do we do when we think they owe us more than we owe them? Is it a matter of waiting for them to take legal action and then putting in a defence using bank charges?
Is there any way we can demand that they stop contacting us until the issue with the charges has been resolved?
What can we do if they sell the 'debt' to a debt collection agency?!
Any tips much appreciated!
Thanks
p.s. we are no longer using the account or credit card and have refused to make any further payments.