Tesco default template removal

[Bebex]

I had a Tesco credit. I did max it out to the full value. I missed 3 payments and they put me to default. I didn’t receive any notice of this and was only made aware when I rung them to amend my direct debit. When I spoke to her over the phone she said I needed to pay the full balance or agree to set monthly payments. These payments were over what I paid on the DD and I have since paid the full balance of. During the call she failed to explain that I had default and have now ruined my credit score and I wasn’t given any written communication or anything on the portal when I logged in to pay. Is there a template I can use to try and see if they will remove it? I now earn triple what I did 3 years ago and I’m struggling to get a mortgage due to this.