A member of my family recently stayed over the limit in a car park next to Asda and received an 'Charge' notice through the post demanding £90 (£50 if paid within 14 days) - all official, with photos of the car entering and leaving. Thanks to the advice of some friends on Facebook, I've now discovered that these are a complete scam and there is no need to pay them. The parking company will go through the process of sending various scary letters from fake solicitors and debt collectors threatening legal action but they never take it - one of the reasons being that they can only claim against the driver and they have no way of knowing who that was, only who the car is registered to.
Parking companies make a fortune out of these so-called charges so don't pay if you get one and don't respond to their letters.
Here's an excellent site that puts it all in perspective (be sure to watch the video of a solicitor showing what to do with the charge notices).