Missed payment appeared on my credit file 4 years after the fact

ACT74

I am going through a mortgage application process at the moment (it was accepted in June but due to poor conveyancer service it will now complete August 10th) so have checked my credit file thoroughly and often when suddenly, yesterday, a missed payment notification appeared on my credit file.
It was from Anglian Water in 2016 when I closed my account due to moving out of the area.
It's only £11.75 which I'm more than willing to pay, I had no idea about it before now.
How should I approach this? What are Anglian Water likely to do? I obviously want this removed from my file, is that possible? Do I need to let the mortgage lender know the details behind this?
Thanks for your help, totally panicking now. 

SpreadableToast

If you did indeed miss the payment and the date of the missed payment is being recorded correctly you don't have any grounds to demand it be removed.  You could ask nicely when you contact them to pay it but if it has been correctly applied they can refuse if they want.

If it is 4 years old the lender may not care, although it might be sensible to contact them to explain the situation.

D3xt3r5L4b

Is it a factual missed payment?

ACT74

To be honest I've not actually seen a bill or anything to this effect so I'm taking it at face value...
I have however now paid it off and they said they "will look into it" for me, as a "gesture of good will" I suppose that's the best I can hope for.
I thought they were supposed to add these within 3 - 6months, not 4 years later?