Success. Following 1/2cwt's excellent "how to", I have been able to get a CoC from Porsche and register my 1997 986 as ULEZ and CAZ compliant. All quite straightforward; the process took about 4 weeks from start to finish.
Now for the last step, to get the 986 V5C updated with my car's emissions data.
Also I have been able to register my "historic vehicle" 924 and MGBGTV8 cars as ULEZ and CAZs compliant. Even easier as a CoC isn't required; just a copy of the V5C.
So again, my thanks to 1/2cwt.