I have a couple of external sockets that are fed from an RCBO which also
provides power (via about 30m of buried 3-core SWA cable) to a class II
Well pump and controller. The SWA armour and earth conductor are
connected to the house earth solely by the CPC in the T&E cable. I'm
wondering about putting a small weather-proof CU at the Well so I can
use its supply for a remote socket, garden lights, and maybe a gate
controller. The question is whether to TT the remote CU (but the SWA
will still have to be fed from the RCBO) or whether to add a 10mm earth
cable between the MET and the SWA armour so I can use the exported
earth. The house has a dedicated TN-C-S supply from a 2-wire overhead
feed, but I have fitted an additional ground rod so (hopefully) the loss
of the supply neutral should not cause a safety issue.

What would the experts do?