How long ago was the IVA concluded and when did it start? Your IP is obliged to keep records for 6 years (might be 7) and should make more effort to locate them --- this could be cause for a complaint to their regulatory body.
Did you keep the copies you were given ?
As long as your IVA provider accepts that you were in an IVA with them at the time, they should be able to at least confirm to the creditor that the IVA was in place, in which case this debt would have been captured under the insolvency regulations. The fly in the ointment will be then confirming the IVA was successfully completed. How has your provider confirmed its completion without paperwork ?
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk