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  Eventually got sick of it all, confirmed with the bank that the cheque was no longer valid and spent the money.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