This happened to me a few years ago in the London Borough of Camden. The bay had no sign displayed so I parked there and subsequently got a ticket. I made representations which were dismissed as they had "no reason to disbelieve the parking attendent when he said the signs were present".
I then went back to the bay in question and discovered a brand new sign had been erected. I phoned up the highways department, they told me the date the sign was placed, which was after the alleged contravention, and then I started the appeal process. However, Camden Council wrote to me prior to the appeal and told me they were cancelling the PCN in light of the evidence I had provided.
I would go back to the bay and see if they have replaced the sign. If they haven't then take photos. if they have, get on to the department that deals with replacing signs and ask them when they did replace it.
The bottom line is that the bay has to be properly signed for it to be legal. Their dismissal of your representations is standard practice. Go to PATAS with it. But get your evidence as detailed above first to strenghen your case.
You can also get more info over at Pepipoo.
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