Stefek Zaba wrote:

Well... TT installations aren't *that* rare, expecially out in t'sticks,
and 100mA-time-delay RCDs are just about always needed there,

Not necessarily time-delayed; the
main-switch-feeding-two-RCDs-in-parallel method has much to commend it.

since it's really really really REALLY rare that you can guarantee a
sub-one-ohm earth impedance with any economically feasible earth
rod(s)...

On a public mains supply you might just as well write "NEVER" there.
The impedance to earth on the supplier's side is allowed to be as high
as 20 ohms (usually is much less of course) - giving a max. Ze value of
21 ohms when you add on a notional one ohm for the transformer and
metallic conductor impedances [see sec. 1.1(iv) in your OSG].

--
Andy