Deduction from Statutory Maternity Pay payments

We have an employee who started maternity leave two months ago. Under the terms and conditions of her employment, she is only entitled to Statutory Maternity Pay (SMP); but in error, she has been paid full pay for the first two months. I am about to contact her to discuss repayment options and hope that we can agree something. However, I just want to check on the legal position should we be unable to do so. Can we deduct this overpayment from her future SMP payments?