Courts have been ruling for years that, as long as there are clearly-displayed signs advising drivers of the charges for parking, an enforceable contract exists between the driver and the parking company.
e.g. In Oldham in 2008:
http://www.oldham-chronicle.co.uk/news-features/8/news/15246/parking-penalty-just-judge-rules
Despite that there have been plenty of people posting on the internet to say that you can ignore such parking charges. That advice has ended up costing some motorists far more than if they'd simply paid up in the first place! (There are even still some people posting that you can get around it by saying that someone else was driving the car. That loophole was closed by the Protection of Freedoms Act 2012).
The matter has now been taken all of the way to the Supreme Court
http://www.thisismoney.co.uk/money/cars/article-3303189/Parking-ticket-disappointment-UK-motorists-Supreme-Court-rejects-chip-shop-owner-s-85-appeal.html
but that still doesn't stop some people from insisting that the charges can be ignored . . .
. . . and it still doesn't stop some drivers from ignoring parking charges altogether - which can get VERY expensive!
http://www.bbc.co.uk/news/uk-scotland-tayside-central-39478203
Unless there are valid grounds for an appeal (such as there being no warning signs visible when entering that part of the car park), I STRONGLY advise paying up!