Is this main earth adequate

Depends - are those black wires also earth connections  or just something unrelated ? (black was earth pre WW2 back when green was 3rd phase, before it became earth)It is pretty dated - probably 1930s, so be careful also that any matching era fuse holders may contain asbestos.
What is the measured Zs ? 
It could be TNS or originally supposed to be water main TT, not permitted since 1960 something.
Is there any evidence of something having been around the cable jacket originally - of that era it could be quite weedy and soldered ('sweated' to the lead). Nowadays a spring connector that combines the spring of a clock with the laceration ability of a corned beef can is the preferred, but the DNO should sanction adding anything like that to their cable first if its not already there under the grot.

If you are in an area with a progressive DNO with records, they may be able to tell you what their policy is on metal cut-outs and may just replace it as a precaution- some will change on sight, others wait to see a fault first.

Other DNOs are not interested and will recommend you add an RCD and convert to TT with rod electrode.
regards Mike.

mapj1 said:

What is the measured Zs ?

That alone may demonstrate that it is not adequate.

What is the size of the G/Y? Is that adequate.

How many fuses are in that service head?

Time for the DNO/DSO to replace that service head with something newer made from GRP as that one might be about 80plus years old.  The caveat is when they replace it you may or may NOT be provided with an Earth.  They should take Zs reading and if the earth is there and working they they must re-provide/maintain it.  My understanding is If there is no earth then the replacement will not have a DNO provided Earth which means you will need to install an Earth rod and ConduDisc

The fuse holders may have both Live & Neutral fuses too.

Cheers!  Clive