Employment law isn't my area, but I've some experience in reclaiming/defending reclaims from payments wrongly made. First off, don't rely on your DH's contract, and don't allow them to say that they can't find your contract, but their standard contract assumes they can deduct overpayments - if they rely on a contractual term to do this, make them prove it's in your contract, not in the standard, not in your DH's, not in a co-worker's. If they don't have the right to reclaim then you don't have to allow it; if they do it without your permission, then back in the day when I did employment law, it'd be classed as an unlawful deduction from wages, which gives you the right to claim damages equal to (if memory serves me right) three times the amount of the deduction.
Secondly, if you have to concede that they're entitled to claim back the overpayments, they've given you valuable information that they're going against their normal practice in not allowing the repayments to be made over the same duration as the overpayments were made. The hidden information in that statement is that they have doubts about their ability (legal and/or practical) to recover the overpayments if you're no longer working for them - that in itself is valuable information, so use it.
In addition, in saying that they will not allow you the usual period of time to repay because they're worried that you'll retire they are also saying - between the lines, but pretty explicitly - that they intend to discriminate against you on grounds of your age. That is, to say the least, fairly dodgy. If I were their lawyer, I'd be doing a facepalm right about now. Good luck.