Problem about student working hour

Hey guys! Need urgent help here!
I was working part-time in a fast-food chain. They pay weekly, and I have made it clear to them about my visa limit of max 20 hours/week. However, in one week, because of a technical problem in their system, the payslip showed the work unit is 21, which exceeded the limit. I was extremely anxious, so contacted the company head office, they explained the payslip had already been issued and sent to HMRC, so couldn't amend anything. However, they issued me a letter explaining that I definitely didn't exceed the time, it was completely a technical error. This letter was stamped and signed by the company's HR office, however, like I mentioned above, the payslip of 21 hours has been issued so this record stays. Since I am trying to extend my visa in the UK, I am quite worried if this incident is going to result in a refusal in my future application. What should I do to prevent it from happening? Is there any office I could contact with?
Please help!! Many thanks in advance!

Hi, don't worry it shouldn't be a problem until unless the UKVI knows about it. But sometimes companies do land up making mistakes and if it just happened once then it should be alright.