Hi Neil
Briefly: The IVA payment will be worked out as your total income, less agreed expenses. Generally there is a small element for emergencies ( I get £20 general and £20 car repairs). If you can make any small saving in groceries, not buying clothes one month etc, you are encouraged to save this for those rainy days!
If you get a pay rise 50% of it will be added to your usual payment, generally at annual review time.
Small increases can be catered for during the course of the IVA -- the secret is to find an IP you are happy working with and keep the lines of communication open. You will find some IP's more communicative than others, and can get a feel for this by cruising around the forum and reading the comments
I would suggest you pop over to
www.iva.com where there is a listing of reputable firms ( with reviews) speak to a few until you find the one you are most comfortable with. The advice ios free and without obligation at this stage. Also, again, scout around the forum. Many IP's post here and you can see how they work.
Best of luck and keep posting .... no question is silly
Reason for edit: amend incomplete link.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014