5 year old final bill from Severn Trent water

Corizzy

Hi everyone, 
I have looked online and cant really find an answer to my query. 
A couple of weeks ago I spotted on my credit file that a missed/late payment had been logged for ST water.
A couple of days later I received a final bill from them through the post referring to a property I moved out of in October 2015.
When I moved out I did all the usual meter readings to all utility suppliers and gave my new address for final bills.
What with moving house (from the Midlands to Manchester), and starting a new job, I didnt keep track of what was being sorted out in relation to the old property, and I eventually forgot about the water bill.
Now I am being asked to pay a little under £300 to ST water as a final bill from October 2015 (received and dated February 2020)
Are there rules dictating how quickly water companies must issue final bills? 5 years seems a bit of a long time.
Sorry for the long winded post and I appreciate any help. 

Cardew

Assuming the bill is accurate it is 6 years for unpaid bills.