Keep a record of all contacts you've made with creditors and CRA's
You don't owe them any money now, the IVA legally wiped out that debt.
Your ex-creditor is trying it on. You have rights under the data protection act. Your next course of action would be to pursue the Information Commissioner, detailing all contact and showing proof your debts are cleared for an assessment.
There are some great little guides and advice on dealing with this problem on his website
http://www.ico.gov.uk/Home/what_we_cove ... ights.aspx
You may have to get to the point of threatening court action but I doubt very much it will get that far once they realise they are dealing with someone who knows their rights under the act.
If you've proof that the IVA has been settled and that debt was included in the IVA then that means the debt is settled and any data that reflects otherwise is inaccurate and you have a right to have it corrected. The data controllers in this case the CRA's have a legal responsibility to ensure the data is correct.
We're willing to wipe the slate clean, give you a fresh start. All that we're asking in return is your cooperation.