Like I said in #13, there are thousands of threads/pages/letters out on the interwebby about this but the law changed on 1 Oct 12 & the vast majority of these pre date that so are now wrong. Even most of the later posts are still wrong as people refer to the stuff they read, about earlier stuff.
There are still many current web sites Pepipoo being a leader but these sites are full of posts of people trying to beat the systems when they know that really they are in the wrong but may wriggle out of a fine through a loophole. I watched an interesting documentary a few weeks back about motorists, some who un-registered their cars so claim they don't have to follow most the motoring laws now. Anyhow I digress, I also am no expert on this but the government guide to the legislation seems pretty clear to me that as long as correct signage is posted then there is no argument to contracts, landowners, fees or anything else.
What makes me laugh about Martin Lewis site wozz mentioned is all the way through it says its about unfair tickets but then uses an example of being 15 min over a 2 hour max parking in a Wickes car park & not to pay it. Well your in the wrong then, your 15 min over, end of. Now I am not saying I think that's right for a 2 hour limit in a superstore carpark and there may be other reasons for overstaying but that website is only interested in telling you to never pay.