I'll say it again in case anybody is listening, I took the advice of faceless random strangers on internet forums and ignored all the demands for payment, I am now waiting on a court hearing to be set by northampton county court for £150 plus additional costs if I lose, which may or may not happen, the only real advice I would give is research as much as you can in to MET and whether they have in fact taken anybody to court/successfully won. Ultimately you will have to face the consequences of your decision.
Some private parking firms are bigger than others, the bigger ones are now taking some non payers to court because they can afford to, if they are seen to win some cases it will open the flood gates to more claims from other parking firms. Their revenues must be down because more people were getting more savvy to it and ignoring the threats of legal action.
The advice I was given last october at the time of my pcn was to ignor, it only came to light this march that all of a sudden they dished out a bulk of 600+ court claims from northampton county court in one week alone, upping their game - I am sure many people folded there and then and coughed up £150 on the spot. Now I have submitted a defense questioning the legality of their charge and no financial loss whatsoever for overstaying a free parking limit, I can only wait to hear from them , they may well give up but I don't know yet as this is current and ongoing.
Just trying to give you a bit more informed advice that may help you to decide.
good luck with it
Simon