From your latest narrative, I would say you have a good case to take to London Tribunals, because you are entitled to rely on the sign nearest to your car. A single suspension sign for such a long bay is totally inadequate; a motorist is not expected to tramp the parking bay to see if there might be a suspension sign on one of the signs.
However, it is all a money-making game for the council. If you decide to take the matter further to the Notice to Owner stage, the discount offer will not be available, (although they may re-offer it when refusing your formal representation). If you take them to London Tribunals it will be with the full PCN penalty in play. It's a double-or-quits gamble, although here the odds look in your favour.