Overpaid on maternity leave

So, I started back at work yesterday and received a letter today that says I have been overpaid £691 (it actually says £1691 but in the workings out it comes to £691 so I am hoping it's that)

It says they intend to recover the amount from my salary, so I guess I have no option but to pay it back. It's just annoying because it wasn't my fault and it says the mistake was noticed in September but yet they are only telling me now!

Very annoying. I'd just negotiate how much you can afford to pay back each month

Read the letter over and over and it contradicted itself over and over so haven't a clue what I owe now. DH spoke to payroll officer and he agreed it didn't make sense and blamed it on the computer. I'm now waiting for a revised letter.