tbh i don't fancy a debate on the rights and wrongs of the csa and having more dc. i think i should explain a bit more about this case in particular before you all think i'm some jealous bitch 
in my case i have been told by the new gf that he does not contribute to her dc, she gets maintenance from her dc father. he is using them as an excuse to avoid paying what he should to me.
he is not good at paying anyway and this, alongside lying about his work circumstances means that my payment has reduced from £134 pm to £21 pm (or thereabouts, waiting on the actual decision from csa)
up until now he has been claiming jsa while working cash in hand £50 a day and while gf claimed as single parent.
i appreciate that by claiming as a couple she now loses her benefits (or tc, whatever she would of been on) but the amount of work he claims to be doing means they are entitled to hb etc.
he supplements what she has lost with what he gets paid cash in hand.
it's so bloody frustrating as i can't prove it. obv i have told the csa all this but there is nowt they can do. they say they will investigate but he isn't lying about where he works, just how much.
bloody gf took great joy in telling me all this by the way, it's not a theory.
i can see why from the op it seems like i'm just pissed that he has gone and moved in with someone else. i wouldn't want his step dc to go without just because mine came first, but he isn't supporting them. we have been seperated donkeys years and i'm expecting again with dp so i can appreciate families who genuinely have to make things stretch further with new/step dc.
i need to get back into the mind set of having £0 from him again. but thats bloody hard when he insists on flashing the cash when dc are with him 