What is the legal position regarding earthing arrangements for a (domestic)
property that has as its main earth, a connection to the income cold *water*
pipe. I know that this is not allowed by the regs (although I think it used to
be?). There are several houses I know of in this suburb area that have this as
the only earth. Does the use of an RCB on the CU make this acceptable or *must*
it be changed?. If it has to be changed, what to? I don't know if PME is
available here - if not how about earthing rods per propery? Although safety
comes first of course I guess this could start to get expensive especially in
that each such arrangement has to be retested/certfied by some organisation?

AFAIK there is no legal requirement to rectify things like this. Such
installations would fail a periodic electrical inspection & test though,
with the need to provide a proper means of earthing flagged as urgent.

Are you sure that you are not confusing a main bonding connection to the
water pipe (which is required) with the actual means of earthing - which
would usually be to the lead sheath of the supply cable (TN-S system) on
an old installation with underground cable feed.

PME (TN-C-S system) may be available (ask the local distribution network
operator), but would require new service cables to be installed.

If the supply is overhead then it's much more likely that there is no
earth. In that case the options are to add an earth electrode (TT
system) and RCDs or to upgrade to PME, if available. A single RCD is no
longer acceptable - there should be a 30 mA one protecting the socket
circuits and a 100 mA one for everything else - see the IEE On-Site
Guide for details.

--
Andy