I am about to lay some 10mm2 CSA SWA from one end of the garden to the
other.

I need to decide whether to export the (Protective multiple earthed) CPC
from the garage via the SWA cable (where a supplementary CU is being
installed) or use an earth rod at the shed & greenhouse at the bottom of
the garden to form an independent earth.

Total seperation distance laterally is 13m but length of SWA cable is
nearer 20m.

And why is all SWA cable all 3 phase? I can't seem to get any 1 phase
cable so I am forced to shove some brown, blue, Green& yellow PVC tape
on both ends of the SWA cable?