First -- dig out your paperwork and see exactly what it says regarding wage increases. It usually says somethimng along the lines that the supervisor is required to review "once in every 12 months" and increases become effective the month after the review. Most IP's take this as a review each anniversary date --- I would ask your IP why they are moving this review forward ( although I suspect it is simply so that they can capture the increase earlier than has been the norm up to now ).
I am guessing what they mean by "what can you afford" is that the increase is supposed to be 50% of the net surplus after the rise --- so, if the gross increase is £100 per month and the IP agrees that your expenditure has also increased by, say £40 per month, the net would be £60, so you can afford 50% of that i.e £30 extra per month. Basically they are asking you to do the work !
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