intrusive questions on council tax form

I am in receipt of council tax benefit. I am disabled.

I have just received a form to apply for this for 2013/14 which is far more detailed than last year and in particular is asking what I do with my DLA ....... and if I use it to pay for a Motability car they want to see all the paperwork.

What the heck has this got to do with the Council and what are my rights not to reply?