When you park on private land where there are clearly displayed notices explaining the charges for parking (and those charges aren't 'unreasonable') then you automatically enter into a legally-binding contract compelling you to pay those charges upon demand.
So, assuming that you actually did park a vehicle on land controlled by Civil Enforcement Ltd (or that you allowed someone else to park your car there and you're not prepared to tell Civil Enforcement Ltd who that person was) and that you parked for the time (or excess time) specified on the PCN, together with a further assumption that there was a clearly displayed notice setting out the parking charges (and that they're not 'unreasonable') you're obliged to pay up. If you don't, Civil Enforcement Ltd can pursue you through the courts for the debt.
What is a 'reasonable' charge is entirely a matter for the courts to decide but one test case went all of the way to the Supreme Court, where it was ruled that an £85 charge for overstaying on a private car park was not 'unreasonable':
https://www.lawgazette.co.uk/law/supreme-court-dismisses-85-parking-ticket-challenge/5051987.article
Ignoring such parking charges can get very expensive, e.g:
http://www.rochdaleonline.co.uk/news-features/2/community/15278/judg
or even
http://www.bbc.co.uk/news/uk-scotland-tayside-central-39478203