I am installing a hook-up for my motorhome on my drive, using a commando socket and armoured cable. At the supply end in the garage I have a 30A RDC and am connecting to the circuit via a double pole switch into a junction box. I understand that apart from the earth lead to socket and switch I have to earth the wire armour at the supply end. Can this be done via the earth terminal in the switch or must it be separate? If separate, how do I do it?
Rather a complex question and not enough information to answer but some pointers. 1) Caravans and cars on charge not stored inside the building should not be supplies with a TN-C-S earthed supply so in theory we should be using a TT supply however this is all well and good in a caravan site where fire regulations require minimum distance between caravan and building but at home extraneous-conductive-parts of caravan and house may be close enough to be able to touch both at the same time or at least not have enough electrical gradient between the two for it to be safe to have different earthing systems. 2) Using RCD protection you need a means to switch off both live conductors both line and neutral as neutral earth faults can mean you can't reset the main RCD when there is a fault even when MCB is switched off. 3) Connecting the earth bond to SWA at the supply end is correct and as you say the earth terminal in the switch is handy and would seem the correct way to go. Where isolated at the socket end as with a TT supply this is really no problem but where the earth is used consideration must be made as to the fault current.
With a TN-C-S supply we hope the earth/neutral in the DNO supply is never lost and it is unusually for a cable to be damaged where phase supply is intact but earth/neutral is lost but not unknown. If this happens it is hoped the supply will trip but before it trips any earthed part will act as neutral return and if the earth is good high currents can flow. As a result minimum size is normally 10mm however you can't get a 10mm cable into a 16A socket and with a TT supply it would not be required.
So with my father-in-law with motor caravan parked on concrete which will not conduct well and was well drained very close to house with extraneous-conductive-parts likely to be exposed on the house within reach of the van although not strictly correct the TN-C-S supply was extended to the caravan.
With my sons house with caravan down the garden from the house an earth rod was used and it has it's own TT supply. The shroud is tie-rapped onto the SWA gland which goes into a plastic box so both earths never meet and the cable was pressure tested to ensure no holes in the outer plastic coating.
In both cases the earth to SWA is small. Standard 1.5mm sq found in the supply twin and earth. But in both cases a risk assessment was carried out by the electrical engineer overseeing the work. (Father-in-law, son, and myself all electrical engineers) The point is there are risks and we have decided these are acceptable.
So your option is do a risk assessment and decide what measures are required or to follow BS7671 blindly and install a TT supply. Without visiting and looking at the installation no one can advise you on what to do. At the end of the day you should be filling in the minor works or installation certificate and it will be your signature for design so you have to do what you are signing to say you have done and design the system.
If in England and not a new circuit then likely you do not need to register the work. In Wales you will need to register the work and it is hoped the LABC inspector would advise as to if acceptable that is what you pay him for he is responsible for safety.
I am sure this is not what you wanted to hear and I am sure many have with double pole RCD protection taken a supply to a caravan without ever having a problem using the 1.5mm earth in the 2.5mm twin and earth and exporting the TN-C-S supply but that does not make it safe or right.