Please don't be offended, but I had to do this so I could read your post without feeling like you were shouting and without it hurting my eyes:
i'm 19 weeks pregnant and split up with the dad at 3 weeks (my choice) since this has happened he has said he refuses to have anything to do with the child because i will not get back together, this baby was planned but because i changed the relationship he says that i can go it alone.
What i want to know is should i push for child support ( i work full time but hardly earn more than i would get on benefits) he claims incapacity living allowance and disability allowance but does a lot of biulding work cash in hand and since we broke up he's back to drinking in pubs every night, is it worth persuing or leave well alone for quiet life ?
It's still early days as to whether or not he'll have anything to do with his child in the future - it sounds like he's still hurt about you breaking up with him.
In answer to your question, if you pursue him through the CSA / CEMIP or whatever it is these days, as he's officially on benefits you'll get all of £5 per week paid to you fortnightly, ie £10 per fortnight.
That's so long as there's no changes to what benefits he's on and he's not paying out for any other children.
As to trying for more money from him from his cash in hand work - CSA will not be interested in going after him for that. My ex did / does (very well paid) cash in hand work, but CSA wasn't /couldn't get interested in it, as the money she earned wasn't legally obtained...