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Just because a service water or gas pipe enters a building in plastic doesn't mean that the rest of the internal metal pipework will not be extraneous. Depending on the construction method of the building (especially concrete framed c/w concrete floors and ceilings) as well as the installation of the pipework, these metal pipes can very easily pick up extraneous potentials from the buildings fabric. That's why you should always test and not assume anything, even when the incoming supply pipes are in plastic....
Indeed, a building's metal pipwork may be extraneous, even if supplied via a plastic incoming pipe. This raises the question, at what point should it be bonded? The requirement to bond close to the point at which the supply enters the building would appear to be irrelevant, as that's not the point at which the external potential is being introduced. In fact it may be impossible to determine why the pipework is extraneous without disconnecting sections for testing.
I recon this happens quite frequently with, say, a plastic water supply and metal gas supply pipe. The water pipes may test as extraneous, but only because they're connected to ground via the boiler and gas pipe. In which case, I would say that only one main bond is required, where the gas pipe enters.