You will need to lodge a claim with their IP (details can be found by searching the person who owes you on the Insolvency Register at: https://www.insolvencydirect.bis.gov.uk/eiir/ ).
If the claim is accepted you will only get back pence in the pound. Depending on the amounts involved, if they have hidden the debt to you from their IP / creditors, the IVA might be in jeopardy if failure.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014