or are we talking about a bonding earth cable completely
separate to the SWA sheathing here?

As I understand it, it must be completely separate, not run in the same
cable at all. It has to be pretty thick, too. I can't remember exactly, but
10mm rings a bell, and it needs mechanical protection for burying, too. If
it gets chopped, it must be relaid end to end, unlike the SWA supply, which
can use buried junction boxes.

Christian.