If the debt was included in the IVA and the arrangement concluded successfully they cannot pursue the debt.
I would contact your former IP and try to get a copy of your completion paperwork so you have proof. You will neeed both the certificate and final report.
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
Agree - if it was included they can't come after you.
As Foggy says, contact your original IP and ask for copies of your original paperwork.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk