Wrongly added Default with O2

[Gurujon]

I ordered an O2 sim card in Oct/Nov 2018. The card did not arrive. A few weeks later, I contacted O2 and told them exactly that and asked for the account to be closed/cancelled. O2 confirmed that the sim had not been activated and that the account would be closed. But they said they had already taken £20 and said they would return a portion. I wasn't going to argue and was glad that it was closed. Several months later in May 2019 O2 registered a default which continued to Jun 2020. I was oblivious to this. No letters or anything like that. Around June 2020, I looked at credit reference agencies. And there it was. I raised an issue with the agency who provided contact details for the O2 which I addressed. After some wrangling, I was able to prove that I did not owe them anything and they said they would remove any marks for the period the default was registered. Which they did. But instead of leaving it there, they registered a default starting July 2020.

This time, they have registered the default with several reference agencies.

I have been rejected on a mortgage and extended borrowing. My credit rating is impacted. What options do I have and what should I do?

They have admitted fault and I have evidence of this

Please help legal beagles