It is a pretty obvious concept that service areas - which are built to serve motorists - should provide parking. Specifically, parking for all vehicles must be provided free for at least two hours, with more detail given below.
A significant portion of formal complaints regarding motorway service areas relate to parking. How can something so simple go so wrong? There appears to be a mismatch between what customers expect and what they are offered.
For specific queries and complaints you should contact the operator, who are likely to ask anybody complaining about a penalty charge to follow the appeals procedure sent to them.
All official motorway and A-road service areas - that is those with official road signs directing people to them - must provide at least two hours free parking for all road users who are entitled to use the facility. This free short stay must be provided regardless of whether the driver intends to leave the car or not. Older facilities may not be able to provide dedicated parking for each class of vehicle, instead relying on a generic 'long vehicles' area.
The two hour free period has been enshrined in government policy since 1979, on the basis that a motorway service area's primary function is to cater for the immediate needs of motorists and that two hours was considered sufficient. While motorway services can make handy places for car-sharing or meetings, this is not meant to be their priority. There is no rule that charges must apply after two hours, but convention is that all operators charge after two or three hours (four hours is common in Ireland). There is no rule preventing services from asking people to leave after two hours. Buying food or drinks does not absolve the customer of the need to pay for parking.
Note that the rule merely states that two hours free parking must be available, it doesn't state that all parking spaces must be free. There have been reports that some petrol stations are charging vehicles for parking.