The claimant hasn't specified if she appealed the DWP's decision, only that she believes they lied.
Before the HB overpayment was created, a call was made to the DWP to confirm the period of entitlement and the end date of 21/05/2016 was given. Perhaps I will give them a call and see if they still say the same period and if they do, I will argue that the OP is DWP Error and non-recoverable.
Thanks for your assistance.
