yes im afraid your being thick

earth wire - sleeved if twin and earth wiring to terminal one(backbox) to terminal 2 with short link (metal front plate)
however, it dosnt amtter what way arround its done
however, to confuse matters further you can take the earth from the twin and earth - or any other cable(exept 3 types of cable your unlikly to find doing the job in domestic) can go straight to the switch/ sockert and not need to earth the backbox as the need was removed for metal backboxes(k/o) with one fixed lug
again if its a metal surface box - like found in garages and workshops ect then the backbox must be earthed as it can be touched(basicly)
cable comes in the following sizes
1-(commonly lighting)
1.5-(commonly lighting)
2.5-(usualy power)
4- (usualy power and water heaters/very small showers, sheds garages ect) - ALSO IF ITS A SINGLE GREEN/YELLOW THEN THIS IS THE SIZE USED FOR CROSSBONDING baqthroom which most electricians dont understand and do it wrong)
6 - small showers and cookers/general power ect
10 - showers/cookers (ALSO WHEN IF ITS A SINGLE GREEN / YELLOW CABLE THIS SIZE OF CABLE IS USED FOR THE GAS/WATER BONDS)
16 - meter tails on small houses/flats and submains ALSO WHEN IF ITS A SINGLE GREEN / YELLOW CABLE THIS SIZE OF CABLE IS USED FOR THE MAIN EARTH TAIL
25 - Main tails on larger houses
35 - VERY VERY VERY large houses/long runs - very rare in a house situation
50 - never ever seen it in a domestic setup
and so on
obviously its a bit more involved that above and you can use smaller/larger cables and may need to use larger cables determind via some nice algerbra :-D
but roughly above is what you could expect in a domestic
commercial is completely different but domestic is pretty much no brainer wiring
as for an rcd -
an rcd must be fitted on all sockets that may reasonably be expected to supply portable equipment outdoors
so basicly, all downstairs sockets and kitchen if its downstairs, cooker point if its downstairs and has a socket on, plus shed sockets / garage sockets / outside sockets(obviously!)
its handy to have on all sockets and showers because of the added safety but preferably not all on one - imagine the fun of finding what circuit in a bank of 20 on one rcd is faulty with no lights....
i completely disagree that its not retrospectivly enforcable in that BS7671 Is not a statutory doccument and is only ued by everyone because it can be used to "cover ones backside" in a court of law should the poopie hit the fan and somone gets hert - if you comply with it 100% then its not actuly your fault.
it is however again different in commercial and public buildings since EAWR and HSE can and do insist on electrical maintenance and ive yet to meet a contractor who wont follow it and makes his own regs up....
anyway, back to the rcd, imagine when you sell your house or somone conducts a pir for your insurance and thers not one, whilst he cant make you have one(anymore than domesticly he can make you follow BS7671(Above mentioned arse covering doccument)) however is he going to sign your certificate and put his balls on the line to say all is well? ofcourse not!!! and you will have to have something done then or have no insurance or if selling have money nocked off by the buyer
and the moral of the story - do it by the book, the agro associated with not doing so is completely not worth it in time expence and liability!
ss