Repayment - overpayment of a grant in 2007

Teacher15

My student loan is finally repaid and I was due a refund which I had to call up to arrange. They then said you have £800+ outstanding separately from an overpayment of a grant. They said the last correspondence was 2007, I said surely you'd have contacted me, it's 15 years! They said they had on 2007 but not since, it had been missed! 

Now I initially had no recollection of this and then think I do remember my local authority saying they'd made an error and ticked a wrong box and I'd been overpaid. At the time I was annoyed as I had done nothing wrong. SLC said I paid 2 x £50 and stopped in May 2007. Now this jogged my memory and I do remember the scenario and being aggrieved about it but setting up a repayment for £50 a month and repaying it. They say they have no other records and I owe it. I contacted my bank but they don't go back that far and have no paperwork that far back either. 

So my question is can they force me to pay this given I have had nothing for 15 years? If I think I paid (my husband remembers me getting a letter saying it was paid) but they say they can't find more payments and I can't prove it being so long ago. 

I asked to speak to someone higher up but was fobbed off to speak to the refund people as I have overpaid. 

Any advice about what I should do? It's so frustrating as I have been paying the loan back since 2004 and finally done with it all to get a blaisé oh but you do owe other money! 

Thanks in advance.