My opinion is that they are on a hiding to nothing; it's a try on. It would be akin to their pocketing a tax refund were your tax code number revised upwards and you entitled to a rebate through the PAYE system. I would simply ignore your ex employers demands, they are just hoping you will pay up like a good little boy. In the unlikely event of their taking recovery action you will, of course, have to retain a solicitor.
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