Am looking into replacing an antique fuse box with a split load
Contactum consumer unit - in part to provide a modern ring circuit.
But a survey of the current installation indicates potential earthing
problemss, so help please!

By way of background the supply is PME - upgraded (new supply cable,
meters etc etc) around 1995-98 from TT(?) after a fault, by the
successors(?) of SEEBOARD.

The incoming supply cable looks quite beefy. It supplies a largish
house plus several small flats via switchgear leading off in parallel
to the main house switchgear (everything goes through the same elec
board meter & cutout). Can only guess as to the incoming supply cable
size, but the cable from the board cutout is 16.3mm diameter overall
(including the brown insulation/sheath), which suggests something
around 70mmsq to 100mmsq depending on the sheath/insulation thickness.
Can anyone identify this size more exactly please?

There's quite a lot of switch gear attached, one of which is a 60A
switch fuse leading to the antique fuse box in question. It is hoped
to leave this switch & the cable (imperial?? - around 25mmsq) to the
antique fuse box in place as it looks healthy. However the earthing
is via steel conduit & looks to require upgrading to a proper 'modern'
copper earth cable.

The yellow/green insulated multi-strand earth-to-water bonding cable
appears to be at least 25mmsq. It has crimped lugs so it isn't easy
to measure directly: the overall diameter (including the yellow/green
pvc insulation) is 10.4mm and it is multi-stranded - perhaps 15+
strands. I can't get an exact fix on the size of the strands, but
they are perhaps 2-2.5mm in dia. Please, can anyone identify the
exact size/spec of this cable?

This is way over the 'typical' sizes shown for PME on page 28 of the
IEE On-Site Guide. Then looking at table 10a on page 161 gives a
wider picture, and it all rather implies the supply must be 70mm^2 (or
larger?).

So what size should the new earth bonding cable from the supply earth
point to the new Consumer Unit be? Any opinions/advice will be most
gratefully received.

If the supply cable is really 70mmsq then am I reading On-Site Guide
table 10a (pg 161) correctly that a 35mmsq earth cable should be
specified for the run earth terminal-to-Consumer Unit? Or should it
be sized according to the 25mmsq cable coming from the switch-fuse
(16mmsq if I read pg 161 aright)?

BTW I enquired about 25mm^2 earth cable at a TLC branch only to be
greeted with astonishment, so if a heavy guage earth cable turns out
to be needed, where can you buy it?

many thanks