The NQMS is designed to be operated within existing quality management systems (often accredited such as ISO140001) which rely upon the concept of peer review/checking and sign-off. It works best if the SQP has an intimate knowledge of both the project in question and the staff undertaking the work. The administering body ensures that there is an external “independent” element that can be relied upon to police the process.
How can an SQP sign off a report and give it the National Quality Mark if it contains historical data collected before the scheme and its quality controls came in to operation?
The quality mark applies to the individual product being prepared (and the team preparing it). The SQP has to consider the reliability of any data collected in vouching for the reasonableness of any conclusions drawn from that data. Any critical uncertainties have to be identified as part of the process.