As with most matters legal, all cases are subject to individual circumstances. However in general, it is not correct to say these charges are unenforceable. These are not criminal matters but as Chris says they are civil debts and can be dealt with in the County Court.
As I said, all cases are judged individually, but in general the claimant has to show who was driving the car when it was parked (action against the registered keeper if he does not admit to being the driver has been shown to fail) and there is also a requirement to show that the driver knew of the potential charges. In this case, bundleone, it seems that both these conditions have been fulfilled.
The suggestion, AIBags that this was not a charge is probably incorrect. The notices usually say something like “Up to two hours – Free; Over 2 Hours - £80”. As Chris says, when you park you agree to those charges. And if you fail to pay you could face legal action. There will always be instances where all the conditions for enforcement of the debt have not been met and these are often cited as examples where these debts are “not enforceable”. But as the Oldham case shows, parking companies can and do use the courts to recover unpaid fees.