EICR codes advice

[davesparks]

So the fact Table 52.3 states that the minimum size for power circuits is 1.5mm2 doesn't mean using 1mm2 goes against the regs? If it does, surely it needs coding?

No, you are not assessing absolute compliance with 7671. You are assessing the installation's suitability for continued service using 7671 as the reference for the standard of safety. You do not code things just because they are 'against regs', solid green earths, old core colours and imperial cable sizes are all arguably 'against regs' but you don't code them as they have no effect on safety.
C3 is to be used to code observations where an improvement could be made for safety, it is not for identifying items which could be changed just for the sake of satisfying a regulation.
If the 1mm is operating safely as it is then there is no observation to made.
If the 1mm is undersized for the load or fault current then it would be a C2 as it would become dangerous under fault conditions.