I recently did a similar task - replacing the incoming lead pipe with
plastic, though being an elderly property there was no grounding wire.
Now the earth for the house is at the back of the property and
water-pipe-in is at the front and as far from the earth pin as it could
be. Am I really required to bond the piping at the house entry point
which will be seriously difficult to get to, a considerable length and
is totally at one extreme of the piping system, or can I take the
bonding wire to say the cold water tank which is central, is a
relatively short run and is easy access.

As I understand it, you really are supposed to run the main
equipotential bonding to the entry point of the services, or to the
point at which they start being metallic. I'm guessing - and it's only
that - the thinking is this is the point least likely to be monkeyed
about with, whereas bonding to something like your main tank is more
likely to get disconnected either temporarily or permanently if
maintainance work is done on the tank or whatever other "less
permananent" bit of the water installation you might choose. And,
pragmatically, that's where an Inspecting Sparks doing a periodic
inspection/house-sale-inspection will look for the main bonding.

That said, the truly important thing for safety is that your pipework is
bonded. If it's more convenient "for now" to run a bond to the tank, or
a part of the rising main or other
"bound-to-be-in-good-contact-with-all-the-pipework" point, I couldn't
blame you for bonding there "for now", while promising yourself to run
the conventional, Regs-compliant bond to the water entry point when some
round tuits accumulate. But don't rely on this random set of bytes in a
Usenet posting, apparently from someone you've never met, in case
someone visiting your place gets electrocuted and you're held negligent
for non-conformant bonding!

You do know, don't you, that the bonding needs to run unbroken (though
it can "visit" multiple bonding points on the way, just don't cut it)
back to the main earthing terminal next to your CU, not to the earth rod
you appear to have (the "earth pin")? Does that make it easier to reach?
I.e., the main bonding conductors to your other metallic services run to
an earthing bar by your CU, and the conductor to your earth rod then
runs from there. Also, that there's nothing stopping you running a
thick-enough (10mmsq, from memory) bonding conductor round the outside
of your house, and back in to the water riser? (Thickness needed here
because it's not mechanically protected, most of all - if you ran it in
conduit a thinner one *might* be acceptable on the TT setup you seem to
have, but I'd defer to someone who knows properly rather than trust to
this random collection of bytes etc etc!)

HTH - Stefek