It depends on why they want this increase. Have you had a pay rise ? Have you had a reduction in outgoings ? Are they recovering an unpaid windfall ? In any of those cases, yes, they are entitled to seek an increase in IVA payments, as per the agreed terms.
Ask them to explain their reasons.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk
Also, have you had a payment break at all? That could also b a possibility.
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