My take on the above situation is that from week 53 to 57 you would use £9811.
I think reg 103 1a tells you to reduce the amount by the o/p from the previous 13 weeks, so for weeks 53-57 this would be £9811.
103 2 tells you that you cannot reduce other by the overpayment from the previous 13 weeks, but at week 57 you are not reducing it any further as you have reached the end of the o/p period in less than 13 weeks.
As the purpose of using this calculating method is just to determine the level of overpayment, so that from week 58, in a “live” benefit claim, you have to use the actual capital then held. 8)
And as for the chances, as Carol says below – about 13-1 perhaps?