After considering all replies, I think I will arrange for there to be a SWA
cable laid underground from the CU in the house, to a small CU in the
garage, with an RCD within, and the lights / power fed from this.

The house system is TN-S ..... would it be correct for the earth to be
exported, or kept separate and derived from a local spike, just bringing
out live and neutral. ?

Thanks,

Nick