Recently found out I have a debt not sure how to proceed.

Several years back I had a credit card with Virgin MBNA American Express, of which I paid regularly every month like clockwork via the online website. Around summer 2013 I logged in to pay as usual, but this time I it said my outstanding balance was £0.00. Thinking this was odd I called up and they also said yes its £0.00. (I have screenshots to prove it too, just in case)

Not questioning it any further I get on with my life. Fast forward late 2018, I applying for credit, of which I get rejected, I'm advised to sign up to Clearscore, I see a default from a company called Arrow Global, etc. Which prompts me to think, how come I never got any letters? I Visit my mums to see if there has been any letters, though everything should be at my current home. Not finding anything I ask my mother, who then remembers "oh yeah there is a few letters for you" and comes back with a bag of junk mail and letters from Arrow Global.

So I contacted Arrow Global who put me on to Capquest to be told, there is a debt but the credit card account number you provided have does not match the credit card account number we have, the advisor then says this is strange, let me send investigate this and get back to you with what we find out. I was also asked to email in my screenshot evidence of the balance being £0.00

Then about 2 months later around Feb 2019, I get a letter saying since you have not provided evidence and we are taking the hold off the account to resume collection. I contact them back via email copying in my original email with the evidence and asking for an explanation of what it going on?

Mid March I get another letter saying

Having reviewed your evidence, we can confirm that this relates to other accounts. The client references are different, your evidence relates to 'xxx xxxxxx xxx' & 'xxx xxxxx xxx' This account is 'xxx xxxxxx xxx'

Arrow purchased your account from Idem Capital Securities Ltd in November 2015. The address we were provided at the time was 'x xxxx xxxx'. Your current address was first traced by Arrow by December 2017 while it was placed with another company called Moorcroft. As Arrow didn't purchase your account from MBA directly, unfortunately we unable to obtain the original account documents...

Far as was aware I didn't owe anything anymore. But now i'm thinking what the hell us going on here?

How does one proceed in a situation like this?