There is a maximum rate of deduction from Income Support, Income-based JSA, Income-related ESA or Pension Credit. The most that can be recovered each week is £11.10 or £18.50 if the overpayment was a result of fraud or a civil penalty has been imposed. Other than Universal Credit where it depends on the claimant's circumstances, there is no maximum for other benefits.
It's not unknown for overpayment decisions to be wrong, there may well be one but the calculation may not be correct. You can ask for a mandatory reconsideration and even if the decision's not changed, you can then appeal against it. There's no harm in trying, eh?