I agree with PP. This doesn't sound like a simple case of overpayment and that you should have know you were being paid. In that case their right to reduce your pay and reclaim the overpayment is not as clear cut as they are leading you to believe.
Do you agree you should have been aware that your pau was higher than it should have been or did you believe it was in line with your job grade?
My employer has salary grades with the minimum and maximum clearly advertised on the intranet but I'm not sure how many people would check if they got a letter confirming a standard increase that it still fell within the grade boundaries.
Do you have written confirmation of the pay increases you received and any restrictions/ceilings to the standard increase?
For example, we had an all staff email that said the annual pay increase was 2% apart from those on more than £40k who would receive a flat £800. In that case if someone on £45k got 2% in error then the employer could claim back the overpayment as the employee knew they should get £800 not 2%.
However, if the grade maximum was £45,500 and the employee on £45,000 correctly received their £800 increase but the employer failed to increase the grade's maximum salary to accommodate this then I don't think they could reduce the employees salary to £45,500 in line with the grade maximum or recoup the "overpayment".
I appreciate your situation might not be as clear cut as that but it sounds like it might boil down to them not upping their grade maximums in line with their agreed pay increases rather than just overpaying you what you were entitled to.