Any appeal window has now passed - so Saba won't entertain any (not that they'd have been likely to accept one within the appeal period, but you would've possibly had a shot at POPLA).
The key potential grounds of challenge are:
- That the site is railway land and therefore subject to byelaws. As such, it is not relevant land for the purposes of Schedule 4 of the Protection of Freedoms Act (PoFA), therefore the keeper cannot be held liable
- That the location stated is too vague, so does not specify the land as required by PoFA
- Any meritorious defence you might have - You haven't told us anything about the circumstances. Was there a good reason for the failure to pay for parking?
Running those first two grounds at the same time could provide challenges, as the first argument requires a knowledge of where the incident took place, which could be seen to undermine the argument that they've failed to suitably specify it (although PoFA requires that the notice specifies the land, that the recipient knows where the incident took place does not mean that the notice has specified as such).
You could craft something up along these lines (sharing a draft if you'd like feedback) and send it to Saba. They'll ignore it of course, citing their missed appeal deadline, but it might help to show you know your stuff.
Alternatively, it's a case of waiting to see if they choose to go to court within the next 6 years. I'm not sure how litigious Saba are.