Hi MM72 and welcome to the forum
Your IP can increase your payments, and is duty bound to do so if he/she feels that you can afford to pay more. You will be reviewed every year, at the time your IP has to compile the annual report to Court and creditors. Seems strange that they say they originally underestimated the figures - what was explained to you at the start of the procedure about disposable income and reviews.
If you have a child, then you will need to adjust your expenditure at that time, but this is best left until the happy event occurs as you will really need to reappraise your finances then.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk