Railway land is now ‘relevant land’ for the purposes of PoFA 2012 (the ticket is dated 27 January I think), so that defence can no longer be used. It is the experience here that APCOA doesn’t “do” court, so this is the one case when you can choose to ignore - obviously come back if they change their tactics.
Otherwise, do you have any defence, there not being sufficient room to park simply means you need to park somewhere else?