You need to see their video evidence. You'll see how to request a DVD in the PCN.
IMO, the following are germane:
The sign does not face oncoming traffic and therefore cannot be read before stopping.
You could not see the sign without either exiting the car or leaning across from the driver's position. Had you exited the car, then making it safe e.g. putting handbrake on with or without stopping the engine, undoing seat belt, checking side mirror to ensure it was safe to exit, getting out, going to the sign, reading it, checking whatever timing device you had, returning to car and moving away would have taken >>> more than 1 minute. Did you exit the car? If not, why and how did you read the sign etc. etc.
In short, your account and their evidence suggests you were stopped for less than 1 minute. Given that the sign does not face traffic and that you are allowed sufficient time to read the sign, then IMO this would likely be considered de minimis by an adjudicator.
But the video will reveal all.