It won't hurt to mention it in a covering letter. Also:
"If you realise that you made a mistake on your online form, you cannot change it after you have submitted the form. Please make any changes in writing on your Completing the application form
If you review your submitted application form online or on paper, you may find that you cannot see some of your answers. Be assured that we have received the answers you have sent to us, even though they sometimes may not be visible to you.
But please feel free to write comments by hand on the printed application form if you want to do so."
Taken from board sticky guidance:
http://www.immigrationboards.com/immigr ... 65576.html
The guidance is from 2014 but I couldn't find anything (relevant) more up to date. There is an FAQ here:
https://www.visa4uk.fco.gov.uk/Home/ViewFAQ
with an email address right at the end of the page.
Be humble for you are made of earth, be noble for you are made of stars.