I'm not a sparky but I think I can answer this. No, you shouldn't use the sheath of swa for earthing. You should use 3-core. The armour should still be earthed at each end though, which it should be if you use the standard gland kit on a metal enclosure. If it's a plastic box, you should use one of those circular brass thingys at each end and connect it to earth.
I always use the separate earth connection for the armour anyway and connect it to the earth bar, even if it's a metal box.
3-core swa will be brown/black/grey. You should sleeve the cores as follows when you use 3-core swa for single phase (from tlc-direct):
Core Colour Changes
Brown is old Red (Live)
Black is old Yellow
Grey is old Blue
Single phase wiring
Grey core is Sleeved or Taped with Blue for Neutral
Yellow core is Sleeved or Taped with Green/Yellow for Earth (should this be black ?)
The above cores must be identifiable