Actually the (railway bye-) law is very clear. These firms cannot enforce fines - only the Station Operator is able to legally do so for Car Parks on Railway land. Just refuse to pay NCP and they will (eventually) go away.They absolutely would but went ahead with it anyway. I imagine the enforcement officer is commissioned, he's there every day without fail now so must earn a fair amount of fines. There will be laws about it, but only about appealing fines. Depends how far you want to take it.
P.S. The Money Saving Expert Forums are very knowledgeable about Car Parking Fines in general. If you ever need help fighting a fine take a look at them.