Hi there,
You sound quite determined to find this guy, so play the long game and 'do him over' properly! By this, I mean that you (and some eager friends!) should:
a) Take shifts in watching his parents place and eventually he will turn up.
b} Then follow him home, discreetly.
c} Come back to his house the next day and follow him to work
d) if you are feeling REALLY sadistic trail him to his social haunts
You will then have his car details, address and place of work and maybe girlfriend / regular haunts details. With that info, you could maybe:
A} confront him and make him pay
B) or humiliate him at work / home with a dramatic scene (tears and baby dressed in charity shop rags would be a good touch!)
c) send the boys round and deliver him some overdue 'good news'
d) inform the csa and do their job for them.
e) wait until he takes his new lady on a particularly romantic night out (the 'jackpot' here is the proposal night!) and make a spectacularly emotional scene that leaves him wishing a big hole would open up and leaves him covered in his meal / drink / both and probably single again!
Be warned, if the CSA get him, he will have to be made to pay MORE than you currently receive in benefits before they will pay you a thing! Crucifying him is one thing, not getting paid for your trouble is quite another!
My advice, corner him and do a deal. The CSA are crap and all the hot air about passports confiscated and tags for non-payers will not put a bean in kids dinner plate!
Good luck!