Just pass this on to your IP. You don't have to worry as all interest and charges will have been stopped. This may very well have been raised before the IVA, regardless of the date of the letter.
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