I need to connect up earth bonding conductors to my incoming gas and
water supplies, and have just checked Whitfield's "Electrician's Guide"
to double-check which side of the respective stop cocks to attach the
bonding.

For an all-copper system like mine, Whitfield shows the water pipe
earthed on the *street* side of the stopcock, whereas the gas pipe it is
on the *house* side of the stopcock (Fig 5.13). I'm assuming that's
correct but can't understand the logic behind it - before I unpack my
earth clamps please can someone explain?!

Thanks
David