Personally, I think £1,500 child maintenance willingly paid per month is, at more than double the CSA calculated amount, generous on your husband's part. It raises questions in my mind as to whether there any reasons for this ? Did he have an affair and leave his ex wife which makes him feel guilty and want to indirectly try to compensate his ds financially ?
Secondly, it is 25% of his net income, but his ex IS also responsible for supporting their son so the £1,500 shouldn't be considered in isolation, but then it is natural that we look into your current family finances........
IMHO, morally, you are responsible for the support of your older twins and that includes being responsible for chasing their father for financial contribution using the CSA, however unpleasant that might be. Have you done this and exhausted every avenue in that respect ?
If you weren't blessed with having found love with a new partner, then, at this point in time you'd be seriously struggling financially as a single parent of 2 teens as a result of your knee injury. Such incapacities, even if short term, can wreak havoc on a family unit's financial security and sometimes end in a very stressful downward spiral financially with loss of home, A move into less than pleasant accommodation, possibly a series of temporary accommodation in a cheaper area, possible forced change in schools for the dc etc. etc. etc.
Happily you are in a family unit with your dh and naturally your dh and you are jointly responsible for supporting the 2 children that you had together between you, I'm assuming, you made the decision that you could afford to support 2 children in addition to any existing financial commitments and factored in a small "what if" safety margin.
I guess what I'm saying is (and please remember that I do feel £1,500 is generous as IMHO dss's mother is responsible for the other 50% of her son's upkeep) count your blessings..... Furthermore, teach your children to count their blessings as it does lead to a happier life if as a child you haven't absorbed an atmosphere of we are all hard done by because of X. Y or Z.