I was overpaid a months salary at the end of my last mobilisation. I put the money in the bank and immediately got JPA to work out what I owed them. Their answer was about £1K more than I'd been paid as they worked on assumption of gross recovery & not net.
My very strong advice is to get in contact with the debt recovery teams, who are usually extremely helpful and see what can be done to facilitate repayment. Be prepared to stand your ground, but be helpful in trying to sort the problem out.
Don't be afraid to knock heads and ask to speak to higher ups, but I've found that when they know they are dealing with a reasonable individual not trying to screw them or run away from their responsibilities, they seem a damn site more willing to help.
Finally, make sure your P60 adds up this year, or you could have some tax issues. You may need to get a corrective P60 issued.