Aren't you in effect doing a L-N impedance check of the supply when doing a Ze check anyway? The only difference is the meter tail vs the earthing conductor however the neutral meter tail shouldn't cause much voltage to be apparent on the MET.
If PME, i guess you could confuse matters by saying that, yes.
You want to measure a loop between the live and CNE conductor in the head - with NOTHING else connected. Remove earths from the service head (or the CU), and switch off the main switch so all you have is the L and the CNE. Check to see what the impedance is, and if it is stable.
Does that sound better?
If you get an OK reading, it could just be that pipework is providing a return path which is slightly lower than the neutral, and so current decides it may as well flow that way too.....The very reason why PME is not allowed in petrol stations - network circulating currents in the metal storage tanks and pipework is not good!