Changing job through ill health

On 14/10/2011 through ill health stress relating problems I had to change jobs I found another job which I started on 7/11/2011 I had to take £9500 less in my salary from £32000 to £28000 I wrote to CSA and explained I even spoken to them on phone few times but it got me nowhere,it took them 5/6 months to work payments out and I still dont know where they get their figures from,on online CSA calculations it state that it is £66 per week which to me £66 x 4 = £264 per month yet they are demanding £310.06 plus £89.13 in arrears,I asked few times Where do they get their figures from ??

And how do they work it out?

I am finding all this is begining to stress me out plus I cannot afford of what they are asking as at the end of the day, Why does it take 5/6 to work it out when they have been told.
Please help