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

Could it be that the earths wired around the cover fastening are earthed to the metal case and that the metal case is earthed to the metalic sheath of the incoming main supply

cable? TNS?

whjohnson said:

Could it be that the earths wired around the cover fastening are earthed to the metal case and that the metal case is earthed to the metalic sheath of the incoming main supply

That would havee been my fist guess - and as above a Ze test should be reassurring or conclusively prove otherwise in that case. Probably originally intended to be TN-S, but as with any mainland DNO these days, treat as potentially PME unless there's specific assurance otherwise from the DNO.

   - Andy.

Looks to me like TNC-S, earthed to the armour of the supply cable, not directly but via the body of the cast iron cable termination and cut out.

Could still be inadequate, depending on the size of green/yellow wire  and on how securely it be attached. And yes, the black wires might be unrelated, or might be old (pre war) earth connections to a water pipe. Earthing to water pipe has been prohibited for decades, but still in practice works well until plastic  service pipes or water mains are added.

I don't know what you would call it but looks like it's earthed well enough under that wing nut it's probably one of those things that's OK until someone starts playing with it then it will all fall apart. I had heard that green was a phase color at one point in time and the comments about the black earth wires has helped me date an old installation I once worked on  it had black earth wires  had no idea it was that ancient tho. Was a 3 phase set up