I've successfully challenged a couple of local tickets in the past, one after having actually paid.
I went on holiday, leaving my car parked outside my flat on a road with no parking restrictions. I returned a week later to find a parking restriction sign had been put up next to my car (no yellow lines) and it had been ticketed.
I appealed, and it was promptly turned down.
Then, a few weeks later, a gang arrived to paint the double yellow lines - but they stopped at the sign, and my car had been parked just beyond it. Needless to say, I requested that my ticket be cancelled and my payment refunded, which they did.
Then, a couple of years ago, I parked my car in its usual spot (with daytime-only charges) while attending an evening class, and returned to find a ticket on it. It turned out that a week or two previously the daytime parking restrictions sign had been replaced with a "Residents only" one. As well as failing to notice that (in the dark), I had failed to spot that the adjacent ticket machine, while still there, had been wrapped up in black polythene.
I invited the council to see me in court and they cancelled the ticket.
I once lived on a main road. There were no restrictions etc. I parked outside our front. Went to work one day and came home to find double yellows all along. The back door led onto a wide entry which, in theory, was OK but some neighbours had multiple vehicles which caused some fun.
