As you have found out, an accurate watch is key to avoiding PCNs for timed traffic restrictions. Most people would consider this de minimis, but councils will enforce to the absolute second, because the money is so good.
I suggest you submit representations concentrating on the minor nature of the alleged contravention.Post up their response when you get it, but don't expect them to fold and cancel. The only unbiased forum for your case is London Tribunals, but you must risk the full PCN penalty there. If you win, you pay nothing, if you lose you pay the PCN penalty, but there are no additional costs.
