The armour of the SWA cable is earthed at the house end, but MUST be
isolated at the shed end.

It is the "MUST" part above that needs explaining. We established in a
previous thread that in a TN supply/TT generator switchover system, that the
TT earthing rod required for generator TT operation may be connected to the
TN supplier's earth. Why does the same not apply here, provided that the
earth rod is tested to below 200 ohms and the RCD requirements are met? Is
it some sort of earth loop current issue?

Christian.