Short and simple:
Dear Transport for London,
The alleged contravention did not occur, owing to the contradictory and ambiguous signage at this location, I refer you to my informal representations in the first instance. While I accept there are double red lines at this location, there is also an upright sign indicating stopping is only restricted from 7 am to 7 pm, and my vehicle was parked outside of these hours. Looking at the location history on Google street view it is apparent that there is meant to be a white bay at this location i.e. a bay that is available at all times. If the restriction was ever amended to be a double red line restriction, it appears you must have forgotten to remove the upright sign, and the result is that the signage at this location no longer complies with regulation 18(1) of The Local Authorities' Traffic Orders (Procedure) (England and Wales) Regulations 1996.
It follows that the contravention did not occur and the PCN must be cancelled.
Yours faithfully,
TFL will probably reject but it's hard to see how they could defend this PCN at the tribunal. Send the representation via https://tfl.gov.uk/modes/driving/challenge-a-pcn and don't forget to take a screenshot of the confirmation page.