Hi, similar thing happened with my husband a few years ago. Although he left his employer on bad terms, anyway, as it took so long for them to find out they had overpaid him, he wrote them a letter asking for a copy of all his time sheets and wage slips, plus confirmation of all rates of pay that were in force whilst he worked there. He told them he would use these to take to an independant accountant who could verify their claim. Surprise, surprise, he received a letter back telling him that although he owed the money, they would not pursue him for it. Id try this first, see what happen, afterall you have nothing to loose