from my experience i hate to say they dont care about your outgoings or what you have already paid, i was doing same asyou and paying her direct for my kids, then got a phone call out of blue one day saying this is csa we basically want money for your 2 kids i explained i already pay her, they just said well we are involved now we will assess your income and get back to me how much i need to pay, its up to me wether i still give her money too aswell as the bit they will take, i was lucky though as i was working through a composite company so where i was earning£5-£600 a week, on paper i was only arning about£150 so they said i had to pay £28 a week, backfired on her big time as i was already paying more