Cable down to the shed

[ppppenguin]

PME, formally known as TNC-S, is where the incomer to your home has the neutral also acting as the earth. Under certain fault conditions the neutral and hence earth potential can rise alarmingly. To minimise this, the neutral is earthed at many points (the ME of PME) along its route from the transformer to your house. I've not heard of CNE but it must mean combined neutral and earth. There is a special case of PME where the transformer only supplies one property and this may be the case for you. There is less risk of the neutral/earth getting to a high potential with this arrangement.

This document explains a lot of stuff about PME etc:
electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf

In particular a PME supply must not be used to provide the earth for a caravan. You need to give the caravan an earth rod and its own RCD.