Hay
once your IVa is closed and your certificate has been issues the agreement between you and the IP is effectively over. You then have a responsibility around who you inform, credit file etc etc but not to worry all of these are easily sorted.
If your debt was included in the IVA it is now done and dusted no matter what rubbish they will try and feed you.
Simply send a covering letter explaining how the debt was included, who your IVA was with, when it ended etc and a copy of your certificate (Copy not original!!!).
Failing that as Harpic says, let them try and take you to court, they would soon realise they will be out of pocket and drop the case!
Most of all please stop worrying, your safe, they cannot touch you
In regards to sorting out your credit file, head over to this areas of this website
http://www.iva.co.uk/forum/forum.asp?FORUM_ID=183 of loads of information about CRA's, how to have defaults removed, how to complain but best of all template letter for you to use courtesy of The Matrix!
Hope this helps
Arty