As far as I know , you would be treated as a family , as you are not legally separated , therefore both your income and your husbands income would be taken into account.
Also , as the maintenance payments are not legally enforced , I believe that any payments , regular or not would not be included in your assessment .
FIS is a payment for families, and single parent families .That is the only two categories that I know of entitled to FIS .
However , I stand to be corrected on the above.