Just to close off the story on this one:
I phoned the council, then followed-up with an email when I didn't get the promised callback.
"Re. PCN GR22852804 – Issued 04/01/2025
As can be seen from the attached email, I appealed this PCN on 16/01/2025 but have not received any response.
I chased this matter with the RBG last week (call ref. PCN-945,732) and somebody from the “back office” was going to contact me, but I am yet to receive either a call or email.
I do notice, however, that the missing signage that means the PCN is unenforceable and should be cancelled, has suddenly been replaced after 7 years of absence. This strongly suggests that my case has been reviewed, but a ruling hasn’t been communicated to me.
Obviously we are now well outside the statutory 56 days within which RBG should have replied and it should be assumed that you have therefore cancelled the PCN, but I would like the official confirmation please.
If you need any further info please call or email me."
I got a reply 10 days later:
"Good afternoon
Thank you for your email regarding the above mentioned penalty charge notice.
Due to an administrative error your appeal was not processed and as you have quite rightly stated as your appeal is over 56 days we are no longer able to contest this charge.
We can confirm, therefore, that the case is now closed.
Kind regards
Parking Services"
I call bullsh1t on the "administrative error" - the timing of the new signage is just too convenient. Still, the PCN has been cancelled.
Thanks to the forum members that offered advice with this challenge - it was much appreciated.