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 916 0647 to find out more about your debts and repayments"
So the claimant will need to ring the Debt Management Service on 0800 916 0647 to ascertain what the overpayment is for, and how much it is. They will need this information to challenge
Landlords can also use this number, or for questions about overpayments being recovered from the landlord they should call the UC Service Centre on 0800 328 5644.
NB Debt Management Customer Service Standards from gov.uk here.