Well if you don't pay it, and you get taken to court for it, and it does happen, you will likely have a CCJ against your record, which i thought was obvious.
It's not simply a case of 'filing PCN's in the bin' any more unfortunately. These parking companies are taking some people to court, and even winning in some cases unless they realise they have a fight on their hands. In other words, you can't just forget about it, you have to ask them to prove that you have cost them what they are charging on the 'invoice', in which case, most will back down. Go over to pepipoo forums and get some proper advice from there.