When you have a payment break and have to pay extra month's at the end do you pay at the rate at which you paid when you took the break or the rate of the final month?
If it was a missed payment and the entire arrears were not cleared by the end then it should only be that amount of cash. If however you had a six month break and the arrangement was extended then it would be at the current rate of your surplus income. Check with your own IP who can clarify your position.