The Citizens Advice website states: “The notice to keeper must be delivered within 14 days after the last day of parking. If the notice to keeper isn’t delivered to you in time, tell the parking company you don’t have to pay the charge.” This is not accurate. Schedule 4 of the Protection of Freedoms Act 2012 requires that the Notice to Keeper is given within 14 days and a Notice to Keeper is deemed as given two working days after postage unless proven to the contrary.
A parking charge that is issued after the 14 days is still valid too so you should not ignore it.
See https://www.legislation.gov.uk/ukpga/2012/9/schedule/4/enacted for the full Schedule 4 of the Protection of Freedoms Act 2012 text.