Who does what? How to get the DNO to correct old works.

[DefyG]

I thought it was VIR tails in steel conduit with the conduit being the earth? This was a normal method for the electricity boards in the days when VIR was in common use.

It could have been done by the electricity board or another contractor at the time, we'll never know!
The point is the OP seems to be under the impression the DNO will correct this just because it doesn't comply now, perhaps some 30years on.
I agree with davesparks, the equipment will be maintained by the suppliers but they will not undertake the alterations FOC the OP wants for his 3 flats.