What was listed on Page 2 (part 7 - observations and recommendations).
On page 1 directly under the "Unsatisfactory" rating, it even says -
"* An unsatisfactory assessment indicates that dangerous (Code C1) and/or potentially dangerous (Code C2)
conditions have been identified."
If only the C3 you mention for a plastic consumer unit, then yes the report should have been issued as satisfactory. Only, C1, C2 or FI (Further information) should lead to an unsatisfactory report.
Having a combustible (plastic) consumer unit is not grounds for a C2 and unsatisfactory unless there is a sign of thermal damage or loose terminals, which do not appear to show on the certificate
If the Engineer is not willing to change their outcome then I would suggest getting that in writing from them, then the
NICEIC would be the place that you could approach to raise a complaint as they appear to be an NICEIC approved contractor.