Free Traffic Legal Advice

The vehicle entered what has always been a free car park for rail users and parked up.  On returning to the car there was a ppn on the window for not paying the required parking fee.  It appears NR started charging for parking back in Nov.  There are signs but not very large signs stating a fee must now be paid and giving a number to call.  The registration number on the ppn is different to the vehicles registration number but not too dissimilar.  Is this pnn still valid.  Many thanks