As a matter of interest, what's the typical voltage of the three wires
connected to porcelain/glass insulators, mounted on wooden poles, that feed
the transformers (often pole-mounted) that step down to 240V for overhead
wiring to houses? I can't find anything on Google, using phrases such as
"overhead power line 240V" or "low voltage overhead power line".

I presume the 240V overhead wiring is 4-wire (ie a star with a wire at each
tip and one at the centre) to allow one of the wires to be designated as
neutral and thus able to be earthed at each house just before the
"electricity board fuse". Does this neutral wire have to be thicker than the
other three because it is carrying the return current for all three phases,
or do the three return currents usually cancel each other out (at least for
the ideal case where all three phase currents are the same magnitude and
exactly 120 degrees apart)?