Is it normal for a Single error in credit report to make a big impact on one's credit score?

I just generated my credit report and it gave a score in the orange. Looking through the details in the report, I see only one "red mark" — so presume this is the only thing that is weighing the score down — against an energy bill that was supposedly paid 6 months in arrears, in 2015.

Firstly, this is not true: I always pay my bills when they are invoiced. This is even obvious from the report detail, as it shows a correct payment every month followed by this spurious entry. Secondly, this appears to be during a time when I was switching energy provider: This is meant to be a seamless process and while experience has taught me that it’s far from seamless, the last thing I expected was for it to mess with my credit rating. Because it was a supplier transition from so long ago, I don’t have any paperwork for this any more; I was only a customer of the original supplier by virtue of it being grandfathered to me when I acquired the property. I’m not even a customer of the new supplier any more; I’ve switched again in the meantime and moved home twice!

Is it normal for just one minor thing — a monthly energy bill on the order of £30 — to so adversely affect ones rating? Given my lack of documentation, how do I even begin going about correcting this?