residents parking bay

my son parked in a residents bay in London less than 1 hour before it expired and was free to all for the rest of the evening and all next day Sunday. Does he legally have to pay or can he ignore it. His only excuse is he had to park as he had paid for Airbnb near by, and there was no where else to park.
thanks 

Sorry, but you've told us absolutely nothing that is any use to giving advice. So please read this and update your thread accordingly.

https://www.ftla.uk/civil-penalty-charge-notices-(councils-tfl-and-so-on)/read-this-first-before-posting-your-case!-this-section-is-for-council-tfl-dartme/

HOwever, a word of warning. Ignoring PCNs and subsequent enforcement documents usually ends in tears with bailiffs at the door.

hi guys he paid so this post is closed.
thanks