The law changed on 31st March 2008.
"PCN parking tickets can now be issued by post if the motorist drives away after the CEO has started to issue it"
http://www.which.co.uk/advice/how-to-appeal-a- parking-ticket/parking-ticket-faqs/index.jsp
Traffic warden and police issued fixed penalty notices have always been capable of being with by summons if the driver drove off before the ticket could be written out. That was not the case with council issued notices and many parking tickets are now issued by post after being detected by cameras.
"If you return to find a police officer or traffic warden in the process of issuing a ticket then, if you drive off, the ticket can be sent to you as they enforce under different legislation. "
http://www.roadtrafficlaw.com/ParkingFAQs(1).h tm
NCP car parking is different again. This is neither the council nor the police/traffic warden but is a private organisation.
They can send you a fine by post but if you refuse to pay they could chase it under the County Court using the Small Claims' Procedure, in which case you will pay costs too.
Traditionally people have been advised that this is too much hassle for the parking companies and they don't bother, but last year a large company went to court and won. This is encouraging other companies to do the same. It is your gamble.