The best thing to do is to apply for change of assessment under reason 8. See the guide for this (
CSA - The Guide - 2.6.14: Reason 8 - a parent’s income, property, financial resources, or earning capacity) because it is quite lengthy to explain, but in a nutshell what this reason establishes is the person's capacity to pay is as opposed to what his/her taxable income is. Obviously self-employed people are a prime candidate for this because their 'capacity' could very well be much higher then their taxable income because they can write off or hide money quite easily. Any knowledge/evidence of the payer's actual 'income' or assetts would be required. There are offciers that do investigations into income but there is no guarantee that a case will be picked or what order in the priority list the case will take. A result will be achieved a lot faster by going through change of assessment.