I once had to claim incapacity benefit, and like you, told the benefits agency when I had been accepted for a new job. I completed the forms and sent them off. 2 weeks later, I checked my account and they'd paid me again. So I called them, and they said they never recieved the form. So this time I went into the office, completed the form and gave them a cheque for the overpaid amount. 2 weeks later another payment went in. Called them again. We never received the form or cheque they said. Had a rant and explained that I had physically given them these things. Filled out another form. Gave them another cheque. Got a receipt for both! Payments stopped. Cheque never got cashed. Phoned them every week for 6 months. Please cash the cheque. Cheque still never got cashed. Eventually got sick of it all, confirmed with the bank that the cheque was no longer valid and spent the money.
1 YEAR later!!!, I get a court summons because I owe them money. Called them, explained what had gone on, said I had proof. Lo and behold, court summons gets cancelled, and I tell them (not ask), that I will pay them back at a rate of £10 per month (it was all I could afford at the time).
So, I did pay it back. After all, it wasn't my money. But they had to wait for it and accept my terms for repaying it.
So I would suggest calling them, explaining the situation and seeing what you can organise with them before you start to panic.
Good luck mate.
Johnz