Fascinating discussion chaps.
The problem (as I see it), is that the earlier editions of the regs state that Supplementary bonding must be fitted in a location containing a fixed bath or shower, no exclusions until the 17th (RCD/MPB/disconnection times met blah blah) were permitted.
The regs and methods of applying supplementary bonding even change within the same edition via various amendments, it seems even the IEE/IET could not agree amongst themselves the best way of doing it.
In one early edition/amd of the 16th all SB had to be connected back to the MET, then it was realised this didn't provide local equipotential bonding and so this practice was discontinued and the more common method of tying everything together was introduced.
I have the first 15th ed, and 16th ed amd 1 & 2 books here the 15th just says it (SB) should be applied, the 16th introduces zones and in the amd1 book the only exception is for Selv supplies, 16th amd 2 adds more information giving examples of what should be bonded.
It would suggest that no one is right or wrong here, as the installation may conform to one or other method of an earlier edition/amd which was correct at the time, so only attract a C3, the most sensible way forward would be to test as Geoff suggested, not ideal but unless you had every edition of the earlier regs to check.
