How are underpayments of UC reimbursed?

From what I understand the Cl has been underpaid UC and is owed a considerable sum.
Cl has an outstanding debt to DWP for an overpayment of another benefit (but for far less than the arrears of UC) but ALL the back payment of UC seem to have been sent to Debt Management to cover this, not just the amount owed.

Question is who pays the balance to the claimant, is it Debt Management or UC and if it’s UC when do they pay it?

I must admit that I have never come across a situation like this before, in other (non UC) cases the debt has simply been deducted from the back payment and the balance sent to the Cl. I haven’t been able to track down anything in legislation to enlighten me and I was wondering if anyone had come across this before?

thanks

My guess is that they don’t reimburse the claimant. They will have been incapable of accepting that there may be a situation where the all-singing, all-dancing UC regime is in the wrong.

These underpayments are therefore, entirely the claimant’s fault and they are not entitled to a refund.