O/P recovery taken as lump sum from ESA backdate.

A client has recently won her appeal and was due roughly a years worth of ESA money. She has just had a letter from the ESA team saying they are witholding payment of arrears while they check into details of a previous O/P. Is this standard practice because I don’t recall this situation before?

She thinks she does have a considerable O/P from some years ago and doesn’t contest it but can she still request the backdated money is paid to her as a lump sum and they recover the O/P in the usual way i.e. as a weekly amount from her benefits?

Thanks.