I called my DNO who said they did not store this information.
I can believe that they won't have records of how the earthing system has been implemented within each property, but what their records
will tell them is whether the supply to the property has been "PME'd" (hence suitable to be treated as TN-C-S).
If I was worried about my earthing I should call in an electrician. If he finds the earth loop impedance too high then the DNO would come out, free of charge, to try to rectify it, although "it was not [their] responsibility".
I think they've missed the point. Given that the installation's earth has been connected to the supply neutral (albeit on the wrong side of the meter), the loop impedance will almost certainly be low, but that doesn't alter the fact that your neighbours arrangement is wrong (contrary to the Wiring Regulations, BS 7671), and possibly dangerous. With regard to the question of 'possibly dangerous', a lot depends upon what I mentioned above. If the supply to the property
has been PMEd, then the only problem is the 'technicality' of the earth being connected to neutral on the wrong side of the meter - in every other sense it is a conventional TN-C-S/PME system. However, if the supply has
not been PMEd, then things are very different - to have TN-C-S without PME is regarded (by the DNO, for a start) as 'dangerous', and unacceptable (probably illegal) in the UK.
If you could find someone at the DNO who knew what they were talking about, they ought to understand the concern. Furthermore, if it did transpire that the supply has not been PMEd and the installation therefore needed to be returned to TN-S, it is really only the DNO who can/should re-attach a connection to their cable sheath (apart from anything else, it could prove a very hazardous exercise, particularly if the supply cable is old).
Kind Regards, John