Your partner's income should only be used to show she is paying her fair share of the bills and your IP should not be taking her surplus. If she refuses to provide any information the IP can assume a 50/50 split of the bills and work out the payment accordingly.
Many moons ago I had a discussion with the representative of a large insolvency "charity" regarding this and he maintained that his firm used total household income to provide disposable income -- whether he was just pulling my strings or not I don't know, but he and I fell out over it !!
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014