You should look at this as the keeper, not the driver.
The contravention.
1. The driver was required to display a BB in the required position.
2. The authority have stated that the contravention arose because the clock was not displayed.
3. There's no requirement to display a clock.
PI
4. They admit that their NOR is defective and they meant that the BB itself was not displayed = failure to comply with the SoS's Stat Guidance = PI grounds.
5. They stick to their guns and the ONLY issue of dispute is whether a clock was displayed. See The Contravention above.
IMO, pl stop researching this matter from the council's perspective. As keeper you receive a NTO, you make formal reps and you act upon their decision, in this case the NOR.