Thank you for the question. The reasons for leaving doesn't really affect this situation. However, under the Employment Rights Act 1996, the employer is legally allowed to retrieve an overpayment but they cannot take it all at once and must do this at a percentage of your pay (unless you agree to pay more). It seems from the email, they have made a mistake and are looking to claw back the over payment over time, as stated, under the law they are legally entered to do this.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
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