The phrase extraneous-conductive-part which means something metal outside is very confusing as if one hammers in an earth spike with a TN system instead of it being called an earth electrode it's called an extraneous-conductive-part together with water pipes etc.
There was a good article on earthing on outbuildings but just looked on IET web site and it has been removed I guess as it was written before 17th Edition came out.
However distance is a major consideration and with garages, sheds and green houses in back gardens of housing estates where the PME system encircles the whole estate there is no need for earth stakes. However where the building is remote from the house with just fields surrounding it there can be a problem with the voltage gradient of the earth and consideration has to be given to using a TT supply.
However to produce a TT supply is not easy as not only do you need to ensure the earth rod you install is making a good enough contact but also the existing rods are also good enough so that the RCD will always protect.
Also of course the connection and surrounding area of an earth rod can have a steep voltage gradient so is always covered so animals (including people) can't touch it.
The RCD measures in and out and if within 15ma of each other will hold in so where on the circuit it is fitted will not matter if current goes to earth rather than completing the circuit it will trip.
The whole idea of SWA is if any thing hit it the earthed steel armouring will both protect and if that fails insure the supply is automatic disconnected so the armour is earthed. However in some cases provision is made to disconnect the earth to pressure test. Common with street lighting to have this so after contractors have been working in area the council can test to see if they caught the cable and damaged the insulation but can't see this being an issue in a domestic installation.
So likely answers are:-
do I need rcd protection in the garage if power supply from house is rcd protected.
Is the body of the greenhouse also connected to earth spike.
Yes if using a TT supply.
Also am I correct in that the swa running into garage is not to be connected to earth in cu and armour has to be insulated.
No incorrect armour normally earthed.
However although likely as explained there are exceptions and must depends on exactly what supply you have, and how far the buildings are from each other. And what other buildings surround you.