Note that although the DWP's Benefit Overpayment Recovery Guide
states that overpayments will be notified to the claimant, it appears that often the claimant will only be notified through the breakdown of their forthcoming UC award on their online account.
It will show as something like this:
"Debts and loan repayments
DWP Overpayment £xxx.xx
Call 0800 850 0293 to find out more about your debts and repayments"
So the claimant will need to ring the Debt Management Service on this number to ascertain what the overpayment is for, and how much it is. They will need this information to challenge
NB Debt Management Customer Service Standards from gov.uk here.