Debt recovery letters are usually sent for unpaid seller fees dating back many months, more maybe then you are able to check for yourself, and easily forgotten.
eBay do chase unpaid fees.
Best to contact eBay Customer Services, they have access to your account and can explain if there are outstanding fees and the best way now to pay them.
Contact CS now, lines just opened, and there's more chance of Dublin answering.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to 10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends.
I recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
@lnathenson