Can I get my CSA payments lowered now that my tax credits have stopped?

I was after a little advice, im currently paying £70 a week maintainance(50 maintainance and 20 backpay), which they had taken our tax credits in to account, we have now had our tax credits stopped as our 2 oldest boys have left full time education, we got over £200 a week tax credits as 1 of my boys is disabled, we are gonna struggle to pay our bills now, im gonna try and get my payment brought down but as from before they want the arreas paying off asap, if i offer them an amount like £5 a week off the arreas instead of the £20 can they refuse if im still offering?