Quote:What's PME? I know (well, I think...) it stands for protective multiple earthing, but what does that actually mean?
That's corrcect, PME is sourced from a TN-C-S supply, which means that the Earth & Neutral are common up to the consumer's terminals where they are then seperated, when you break it down you see that the C n TN-C-S is common as in both neutral and earth are one conductor, and S is for seperated, where the earth is seperated from the neutral at the supply point.
Quote:Does each socket need earthing to the structure of the cabin or does the earth in the cabling take care of that?
No, The earth from the circuit should only protect the circuit, but you would need to properly investigate what the bonding requirement's are for the porta-cabin, all this would be supplementary to the circuit earthing, or circuit protective conductors (CPC's)
Quote:I can't see any evidence of an earth rod on any of the three cabins that are all wired together (although it is very dark and dingy underneath, so I might have missed it). Should there be one?
I would think there should be one, and there probably is, this is usualy in the style of a small black box which is visible from the surface up to about 2 inches, sometimes more, and will or should be marked with 'earth - do not remove'
Quote:The old wiring has never given any problems and, as far as is known, is correctly wired up
It probably is, however testing & inspection would be the only way to confirm the safety of the installation, however I have no doubt you have correctly wired it and are more than happy with your arrangement.
Quote:the body of the building should definately be main bonded to the CU in there and it is probablly best to avoid conecting it to a PME earth (use a seperate rod if needed)
Definatly