It's a bit more complex than that Rob but firstly they have to have written authority of the land owner (typically supermarkets and the like are on rented sites so they are not the land owner), they then have to provide a genuine pre-estimate of loss which they base their charge upon. This latter point is the one that is tripping them all up at the moment. They cannot include their imagined 'fine' element in this pre-estimate so if the implied contract (the sign you drove past) is correct and you get two hours free, you overstay by 20 minutes, the loss by them is £0.00 eerm... They have to pay DVLA to recover the registered keeper's details to be able to send their speculative invoice; if you get one don't admit to who was driving and they have nothing to work on, they will send ever threatening red letters that will cost them each time but they will soon give up. They work on numbers, if they send out 100 letters asking for £80 reduced to £40 if you pay within 14 days and 20 fools pay them they are quids in by a country mile, another 10-15 will pay on the next red letter and so on... They are on very dodgy ground indeed and eventually they will get their comeuppance but in the meantime don't ever pay them and don't let your poor old mum pay them either, they are nothing but blood sucking leeches.
Local authorities are in the firing line at the moment and Eric Pickles is gunning for them, in particular their over zealous use of $camera cars and CCTV for parking and bus lanes etc.