Sparks wrote:

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!

S'far as I understand it... for one thing, there's a balance of risk:
the possible differences in potential between the supplier's earth in a
TN-S setup and the 'local' earth of buried metallic services doesn't
arise in 'normal' conditions, but rather in the case of faults, while
keeping all your inside-the-house pipework at the same potential
protects against likelier, local faults. ('Better still' from this p-o-v
is for the incoming services to be non-metallic, so it's just the
within-house pipework which is bonded.) A second factor is how low a
reistance the local earthing has: the connection to earth that you get
from a substantial length of buried cast-iron water main and gas supply
is usefuly lower than that from one lil' earth rod.

But I've neither read deeply nor thought hard about this, so I'd welcome
anyone better-informed weighing in...