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As in the Directorate ships in

http://mannmomo.blogspot.co.uk/2015/01/return-of-overseers-at-spartan-games.html

Is there some kind of mask or filer or something that makes this easy? Or maybe its decals? Or is a perfect hexagonal grid really a matter of a very steady hand and impeccable geometric sense?

Spray black. Find hexagonal netting you get on some bags of fruits like oranges. Use that as a stencil and spray grey. Should give you a decent effect

I believe there are stencils for just such an effect but I can't recall where I saw them at.

anarchy stencils

Could be done with decals, but on anything except a small, flat surface, they'll be an absolute nightmare to put down. Stencils/masks are the way to go - I'd wager that's how your example was done. Different companies sell them (and in different scales), but you can also make your own, if you're so inclined. It'd take some time, so I'd only do it for a one-off project, but tracing over a printout of your chosen design with a knife would let you cut a decent, if not perfectly precise, mask out of frisket. How much is your time worth to you?

Stretchy, reusable stencils:

http://www.anarchymodels.co.uk/index.php?page=shop.product_details&flypage=flypage_new.tpl&product_id=163&category_id=27&option=com_virtuemart&Itemid=1

That effect is indeed airbrushed with a hexagonal mesh stencil. White(grey maybe) over a black base.

You could get it in rigid mesh, but for models flexing it around the surface is going to be best. You'l want it to be self adhesive, you could spray repositionable spray glue over one side to achieve that on a non sticky stencil.
If its not stuck down smooth and flush when you spray over it, the edges wont be crisp, spray lightly, and peel it up (carefully) before its dry to avoid tearing the paint film edges.

Thanks!

Time to invest in some stencils, I guess...