BS. The only thing that results in an unsatisfactory result is a C1 or a C2. Professional sparks should not be looking for any opportunity to do a complete rewire either, the existing installation may be easily corrected by fixing the faults found. If you are going to issue an unsatisfactory certificate then you need to state exactly why it is so, not just give it an ambiguous "FI".
No. Professional Sparks job's are to make sure the installation is safe.
The selling of your services are completely unrelated to the issuing of certificates, but I do see EICR as a sale tool, because they are..
That does not mean you scare people in to having work done which is not required.
Any rewire will have pro's and con's, and its the customer choice, your job is to explain it and sell your self.
But the us of an FI as,
"well I don't want to put a nasty C2 down as Im' unsure,... so I think if I do a FI we'll all be okay and HappY with a Satisfactory certificate"
No I'm sorry it unsatisfactory it not Vague at all.
you also need to think your issuing a certificate say "this is up to standard and Safe"!, the further investigation is really, I have run out of time on( as your on time frame to complete the testing) this but I am very unhappy with this, it need to be investigated to identify the problem and made good.
Just out of curiosity how many C3 would it take before You issued an unsatisfactory report? ...