From the 2 statements as quoted I suspect the Regs mean you can use 3 core
+
earth (looks like twin and earth) rather than FP200 or Pyro. They don't
appear to mention a size. BRK suggest 1.5mm probably because the
interconnection will be low voltage (especially if the mains fail and
you're
relying on battery back-up) so you don't get a large volts drop over the
length of the run.
Richard.

Thanks, Richard. I wouldn't have expected the voltage drops to be
significant -- the currents will be in the signalling rather than power
range????