Yeah IP's take fees from what you pay them, they usually take the form of a set up fee and an amount per year. They form part of the IVA agreement which the creditors agree to. If they dont like them, they will modify them.
We dont own our own house so have never had the mortgage issue.
However I seem to recall:
1. Have you switched to interest only? Dunno if this is still viable but used to be wanted?
2. I seem to recall something about how much income is "reasonable" to go on mortgage? £1.6K might be seen as high? Especially if you could rent somewhere for less?
You can always shop around for a IP, and go with whoever you feel best with (see
www.iva.com - has lots listed, and reviews etc).
At the end of the day you may have to live with your decision for 5 years.