Under the Electricity Safety, Quality and Continuity Regulations, 2002, the DNO should now provide a suitable means of earthing with all new installations. This is unfortunately not retrospective, in other words, they do not have to do the same for existing installations. In previous dealings with DNO's, responses have been mixed, some will attend if they feel there is a fault which they can rectify, others may just tell you to deal with it! In extreme circumstances when the Ze has been almost so high as to make it unuseable, I have even had to instal an earth electrode and RCD, and treat the system as a TT. You are miles away from that position!
In a true test of Ze, the installation should be isolated and the earthing conductor removed from the MET before testing. This removes all the parallel paths afforded by the protective bonding conductors and gives a 'worst case scenario' value for Ze. Obviously, when Zs values are measured, the earthing conductor and all the bonding conductors are in place, because we now have a live system. The re-introduction of all these parallel paths can often greatly reduce the Zs values, sometimes even lower than the Ze value! This is one good reason why we should not normally 'calculate' Zs values from Ze + R1 + R2, as this often bears no resemblance at all the actual measured value. (I only ever use calculated values on 'dead' installations waiting for a supply to be installed, purely to appease the DNO who want to see test sheets before livening up). And please do not use a 'bonding clamp' around the cable, they are not designed for that. Bonding clamps must only be used on solid pipes, they are not to be put around cables, armoured or otherwise.
If you feel that the earth connection is loose, or in danger of becoming detached, then lean on the DNO a bit and tell them that you feel it is dangerous. They should respond, and perhaps solve your dilemma. but if your Zs readings are still too high, you may have to consider RCD's ? Good luck.
