You cant change IVA companies im afraid.
If you and your wife are splitting up, is it still possible for you to both pay the contributions between you?
Its normal for the first payments to be used to pay the fees.
I suspect the letters you refer to are from the iva council?
This is a company that trawl the insolvency register for peoples details.
They send the same letters to everybody in the hope of "helping" you declare bankruptcy (for a fee of course)
If you search on the forum you will find lots of posts on that subject.
Sit down with your wife and work out if it can still be paid between you.
http://www.insolvency-service.co.uk/newipsearch.htm
If you follow that link, you can find a direct email address for your IP.
Send an email and outline any concerns you may have.
Hope that helps