Firstly attend the hearings alone then it wont cost a penny.
Caps deal with enforcement of AoE orders but its the judge who sets what should be payed and the protected earnings level.
The payer would receive form N56 which is basically a statement of means income and basic needs, they then send form M338 to accounts to confirm his income.
So now the madness, a judge can agree you need x amount monthly and order it, but if N56 shows he cannot pay at that level you get the problem you have, what is very important is to confirm with court the protected earnings level is correct.
A problem we have seen on wiki before is the judge makes an order in court orally dictates it ready for typing, and then by mistake the protected earnings level is actually the stated needs and not the level decided by the judge, easy to rectify via the slip rule though.
As far as the arrears are concerned you need to ask for a "full order" meaning he will pay all the money owed, or you may have the choice of accepting a lump sum if offered.
In order to be sure of having a full order you need to be reasonable when asking how much per month he should pay for the arrears, again this will depend on his statement of means,its also important when considering his statement you look at what he actually needs and not what he says he needs.
If you talk to caps they will tell you they rarely if ever receive N56 with a stated disposable income, most state they are short every month and thus should not have to pay.
Its worth looking at N56 at HMCS so you know what is reasonable as far as court is concerned.
The payer has the right to apply to vary or discharge the order should their circumstances change, if the payer is out of work and therefore cannot pay enforcing is pointless, court cannot make the payer pay what they don''t have, and no costs order would be made.