I think one of the problems, more so with the larger enterprises, is that the running of the IVA is dealt with by a team of office workers whose knowledge seems to be variable in the extreme. When questioned or a request is made to speak to a manager or the IP they "dig in", rather than refer.
I am fortunate, so far, in that the team dealing with my IVA seem very competent (however, I have had no need to test them yet). That said, I have never had any contact whatsover with my IP ( despite trying).
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014