Do you accept that the employer has miscalculated the wages?
What is the amount of the miscalculation?
Were you aware of this before they brought it to your attention?
If you are aware that you had been overpaid and you ignore the issue and spent the money, that is theft. It is not theft if you were not aware that you would been overpaid and spent the money.
They actually have 6 years in contract to pursue you for any overpayment.
However, if you believe this money was yours and you had no reason to suspect there was an overpayment, you are having to repay through no fault of your own and it is not unreasonable for them to give you a reasonable amount of time to pay the money back.
That would probably be about £100 per month. Expect them to say that it is not acceptable and they want the whole lot in one lump but ignore their protestations and just send them the £100 per month or, better still, send them 7 x Post Dated cheques dated one month apart each for £100.
If you dispute that they have overpaid by this amount would agree a lesser amount, pay the lesser amount and if they feel that they are entitled to more, they are going to have to raise the issue, in the County Court in necessary and you would have the opportunity to defend the action.
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We can still exchange emails. Best wishes.