If no bonding is made to the water/gas services then yes, the metallic path via the armor back to the sub-station will be the only path for fault currents.
Even if direct bonding isn't present, however, it's quite likely than some other parallel earth path will exist. For example, if there's an immersion heater for providing hot water, then the ground wire from the dist. panel to the heating element will ground the copper cylinder, and on an old plumbing system with all metallic couplings, this will quite likely provide an indirect ground path through the incoming water supply pipe.
Under this system, the neutral is linked to ground ONLY at the star point of the xfmr. From there on it's kept separate.
Yes, black is reserved for use as neutral in British wiring. For anyone who hasn't followed the color code topics elsewhere, I'll post a separate summary of U.K. colors in this area.
[This message has been edited by pauluk (edited 10-13-2001).]
