MadJen wrote:

You had better check it is pme - ie the supply company provide you with an
earthing point and not a TT (with an earth rod).

It sounds as if the OP has a TN system, i.e. the DNO does provide an
earth connection. This might be either TN-S, where the earth comes via
a separate conductor (the sheath of a lead sheathed cable, or a separate
core in a concentric service cable), or TN-C-S (=PME) where the service
has a combined neutral and earth conductor.

If you were moving the box I would have been tempted to replace it with a
new one

A "new CU" was referred to, so that may have been done.

Each supply authority has its own requirements for the maximum length of
meter tails whether joined or not [...]

The maximum length allowed will be either two or three metres. This is
the total length between the supply terminals (i.e. the meter output
terminals or the output terminals of any isolator provided by the DNO)
and the consumer's main switchgear.

If the total distance between the supply terminals and the input to the
consumer unit exceeds the limit, fusing must be provided on the
consumers side of the supply terminals within the 2 or 3 m length limit.
In other words the OP's unfused isolator is OK if the total distance
to the CU is within the limit, AND all sections of the tails are sized
in relation to the supply fuse (= 25 mm^ for a 100 A main fuse or = 16
mm^2 for a 60 A main fuse). If the total distance exceeds the limit
then the OP will need to replace the isolator with a suitable
switch-fuse unit, fitted with a 60/63 A or 100 A fuse as appropriate to
the assessed max. demand of the installation. Note that the wiring
between the switch-fuse and the CU is not then part of the meter tails,
but is a distribution circuit, which needs to be designed in accordance
with BS 7671.

Yes you can extend it using an earth block - and if you have an unearthed
installation I would do it quickly!

Agreed, although a better alternative might be to replace with a single
length of 16 mm^2 earthing conductor. (NB: if a metal-clad isolator or
switch-fuse is used that will need its case earthed, so the earthing
conductor would then go via the isolator/switch-fuse's earth terminal,
provided that there is sufficient terminal capacity.)

There has been no mention of the main bonding conductors in this
discussion. Where are those connected at the moment?

[...] as the work you have listed requires certification under part P

This is true, but only in England & Wales.

--
Andy