1. It should refer to bonding not earthing. If this is missing it should be a C2. Unless testing has proved bonding is there but it cannot be accessed then C3.
We can only make this assumption, we have no idea how the gas and water is presented. Both could be supplied via plastic pipes.
2. All circuits are RCD protected so this is not required.
Again without doing the testing this can only be assumed, even with RCD protection certain conditions have to apply.
3. A 2.5 radial should have no larger than a 20A fuse protecting it. As it is a 32A this should be a C2.
You are both right and wrong, Clipped direct 2.5mm twin and earth can carry 27amps, however designers can push limits to suit diversity and length of time the cable will be at full capacity. I would also hazard a guess the the cables are imperial and therefore not 2.5mm but larger. Common practice on a 30amp breaker in my day.
4. No code.
Agreed, not part of BS7671
5. As point 3 above.
As above, however we have also been informed that there is no immersion heater, so the whole certificate is invalid.
6. The main head has a 100A fuse installed so you would want to see at least a 100A main switch installed on the CU - C3.
The picture of the cert is not very clear but it looks like 80amp to myself. Regardless this sentance makes little sense, for instance a 32amp ring circuit is protected at source by 32amp, the socket outlet (load) is protected by 13amp. Same principle.
It also appears a front end RCD has been installed that means in the event of a fault the whole installation will trip. This is not ideal and he has not picked this up. I would code this depending on the property but would be a C2/C3.
This would never be a C2, there is no danger, just inconvenience.
But you can ask 10 electricians these questions and get 10 different answers.
Agreed.