Student maintenance payments

Boofey

Hello, My son is currently at University and we have been helping with his maintenance load payments. previously both myself and my wife were working full time and he has been receiving the minimum amount and we have been paying the rest. This year we have been assessed on our previous tax year however my wife is now no longer working due to cancer treatment so is on ESA as a result and I have retired so my pension income is much less than my previous salary. Clearly we have dropped our income above the required 15% for re assessment so I intend to complete the re assessment form. 
As part of my pension I received a lump sum. I was taxed on a portion of this lump sum. On the form it asks for any lump sum to be declared but only asks for the amount that I paid tax on. Does this mean I only put down that amount and not the full lump sum?

Has anyone gone through this re assessment before and if so how does it work? As we pay the full amount I cannot see it being any worse after re assessment but I am worried they will want even more support. Any advice would be appreciated.