Help, billed for overpayment of childcare vouchers??

[childcarevouchersargh]

I left my last job in August 2017. I've just received an invoice for £676 for overpayment of childcare vouchers as they claim they continued paying them to me till Oct 17.
I have no idea how to access this info. Please be kind to me, I have ADHD and managing finances etc is a huge challenge for me, always has been.
I have the voucher statements from my new employer that shows that my first payment was received in September 17, then October and ongoing. Surely I couldn't have been given double vouchers on Sep and Oct that year? Isn't it a tax / HMRC thing?
So there may well be £676.41 worth of vouchers sitting somewhere that I didn't claim (as I was no longer being paid by that employer), and they are welcome to them back.
But at the moment I just have an invoice for that amount that needs to be paid by 10/8/21.
With no contact phone number or email address obviously
Oh and the employer is Barts Health, a bloody massive NHS trust...

Now I've calmed down a bit I've managed to get into my old account and sure enough there it is. I'm going to call the union on Monday. They can have the money back but they need to get it from my voucher account not from me. It was their error. Bloody hell I really don't need this.