I've run some 2.5mm SWA out for garden lights.  I chose 3 core, using the black as earth, because as an amateur sparky I wanted to absolutely sure of earth contunuity. and didn't want to rely on the armour.  I've heard anyway that the armour degrades and loses its conductivity over time.  

I'm fitting SWA glands and shrouds for entry into the weatherproof plastic JB's at each end.  I'm assuming I don't need to worry about an earth tag (drilling through the box, inserting bolt for earth connection and reducing the IP rating of the box in the process), nor piranha earth nuts, given that I'm using one of the conductors as earth.  Is that right?

Pretty sure I know the answer but I'm a cautious sort of bloke.

Technically, it depends on how it's terminated - if the armour has been cut back before it enters the enclosure, there's insulation & sheathing between it & the live cores, so it's not an exposed-cond-part.

Despite that however, you should always earth the armour to provide automatic disconnection should the cable be cut through.  The pirannha nuts will save drilling a seperate hole for a brass bolt.

Many thanks for all the helpful comments.  I'll earth the armour to be on the safe side.  

As I've already got the earth tags supplied with the glands I'll just go with the drill hole and bolt option.

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Via a stuffing gland.  Sorry - just realised all terminations will be outside so not a good idea.  I was thinking about within a building, where a gland is often used at one end, and the armour cut back before it enters the enclosure then taped up at the other end (think CU in shed or garage).

Glad someone does mate!