I have no constructive advice, except that the CSA are a bunch of useless twunts, who couldn't organise a piss-up in a brewery, even if the beers were placed in their hands.
The CSA take SO long to process a change of circumstance (like your Ex getting a job, and therefore coming off benefits), that by the time they have finished processing the Change of circumstances, there has been ANOTHER change of circumstances. So they are forever chasing their tail, while the RP gets sod-all maintenance. A figure 'added' to the arrears is worth precisely FUCK ALL if you know it's never going to be enforced or PAID.
Though, if you put in an official complaint to the CSA, and 'escalate' the complaint - you CAN get them to try enforcement action. They CAN send in bailiffs (though they don't like to), they CAN deduct an amount from the JSA for arrears (why aren't they doing this?) - If he doesn't need to pay 'current' maintenance due to the reduction for overnight stays, then that £5 SHOULD be put towards his arrears. You need to get in quickly though, if he has other debts that may be being taken out of his JSA, then they can't take more than a certain amount.
IF that IS the case - you can put in a complaint that you should be getting £5 a week for the arrears he owes, and as you aren't, you EXPECT that bailiffs will be sent in to recover the maintenance debt, OR for you to start receiving £5 a week deducted from his JSA to pay the arrears.
Seriously, I'd put in a complaint. I have done this before - long story, but I ended up getting half the amount my Ex owed my DD as compensation from the CSA for their 'maladministration'. Never going to see the other half of the money, granted, but it helped to pay off SOME of my debts that I had incurred through caring for DD, like childcare costs that had mounted up.