thank you for your comments where would I get a copy of the TRO pertaining to the area? From Bexley council who issue the PCN?

Our car was parked outside my friends house - Oaklands Road, Bexleyheath. There was a parking permit clearly visible on the nearside passenger door. I have parked my car here for the last 13 years. My friend had not been informed of the change of display regulations. I only found this out by phoning the council office that it must now be displayed clearly on front dashboard. Having parked in the same way for such a long time one does not study the ticket which my friend had purchased to supply to her visitors. I cannot believe that a warden could have missed seeing the permit. I accept that regulations for display have changed but surely this should have been made clear to the purchaser of the permits. I also fail to see how a penalty can be incurred for displaying in the wrong window. Any thoughts?