Hello and welcome Gary
I think the advice others have given you is sound and the best way forward. I can only suggest you try hard not to panic. I was in a similar situation about 5 years ago when they suddenly sent a letter saying I'd had an additional income for one month of £2400 and that meant I wasn't entitled to rent/council tax benefit for 3 months and I owed them £2600. It was their mess up. I had been backdated tax credits that were owed to me and because of a change in circumstance I had been paid £2400 in one go by HRMC... but on the same day that payment went in to my bank I paid HMRC back £1700 that I recieved and was not entitled to before my claim changed! It took the housing office 7 months to even look at my letter of appeal because they were back logged but when they eventually did they had to eat humble pie and admit i didn't owe them anything! I spent that 7 months worrying sick as I've never had any debt and never tried to defraud anyone (which was what they implied) but ultimately it got sorted out. Even if I had owed them anything they would have had to take it at a minimum amount of a period of months and not in one go.
There are a lot of advice lines out there so give the ones suggested a call and try not to fret too much.
take care
Debbie