If you are a fulltime working mum, then I dont think you can get any extra benefits.
The only other thing you can do - if you wanted to go down that road, was to take your ex to court so that maintenance goes through the system and that you would get it on a weekly basis. Even if he is on the dole, there is no reason why the judges would exempt him from paying nothing, or why he himself should not pay anything. Not sure it is worth all the hassle though - but if it gets tight and you cant afford to feed your child, then extreme measures need to be taking.