I received a private company parking ticket after the pay and display ticket I bought fell off the dashboard (yep, the one time I didn't bloody check it, it happened). I appealed, and surprise surprise, the company - Excel - have upheld their ticket.
Unfortunately, it was at this point I started remembering about how private parking tickets may be unenforceable and started searching on here and on MSE.
It would appear that they probably are unenforceable, and it's up to you if you want to risk the "probably".
I've read and re-read what makes for a "fair" parking ticket, and added on to that my experience of working for a council parking department many many moons ago. I think that as I did have a valid ticket, albeit not correctly displayed, I should not pay the parking ticket.
My problem is that I convince my self of this argument, and then get all nervous and worried I'll get caught out somehow - damn this conscience! So, is my reasoning reasonable? And do I just ignore Excel, or write back to them one last time explaining my reasoning and then ignore? Finally, worst case scenario - they take me to court and I have to pay ... the original £100 fine and £25 costs? or could there be more that I end up being stung for?