As the title really, and as you can see in these very poor screenshots.

I need to find one for a fancy dress costume but don’t know what to search for. Harness? Yoke? Suspenders? (what could possibly go wrong if it’s the latter).

It’s a harness of some sort, possibly adapted from a climbing harness, or maybe military or security services combat rig.
It could even have been custom made by the costume department, all the crew patches are custom embroidered, for example. Really difficult to say, after all this time.
[edit] this might help a bit: http://www.imfdb.org/wiki/Aliens

is it a gun/ammo holster?

Good keyword for you is webbing.

I guess ammo if anything. She carries her rifle and flamethrower on a sling, so no holster, I don’t think.

Looks a bit like a steadicam harness.

after a quick google “rig” seems to be the correct name:

http://www.opticsplanet.com/tactical-assault-gear-phalanx-chest-rig-tactical-vest.html

Ripley toy: http://www.onesixthwarriors.com/forum/sixth-scale-action-figure-news-reviews-discussion/680192-hot-toys-aliens-ellen-ripley.html

Google would suggest it’s an “ammo harness”.

The oracle for (often scarily accurate and expensive) cosplay / fancy dress outfits is therpf.com; I got the name ^^ from this thread here.

someone on the next pages says bandoiler

quick google and:

http://resin-movie-reproductions.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/a/l/aliens_uscm_bandolier.jpg

ie the things you see in westerns and Arnie wears in terminator

Tactical sewing – now there’s a hobby!

If you want to get really tricky, you could up the stakes by building your own 1:1 power loader:
http://www.instructables.com/id/Life-Size-Power-Loader-Costume-from-Aliens/
😀

Cougar – Moderator
Looks a bit like a steadicam harness.

Modified Stedicam harnesses were used for the big smartgun rigs.

M56 Smart Gun

Both PFC J. Vasquez (Jenette Goldstein), and Private M. Drake (Mark Rolston) use M56 Smart Guns. These were made from German MG42 machine guns. The body-mounting apparatus was made from Steadicam “arms”, with other miscellaneous parts coming from various motorcycles, most notably the handlebars used for the weapon’s grips.

There might be something in Flecktarn’s range that you could adapt:
http://flecktarn.co.uk/militarywebbing.html

Hah! Ace.

someone on the next pages says bandoiler

I thought that was just a single-strap thing, like Chewbacca wears. Didn’t know it was a generic term, every day’s a school day.

Bandolier is a single over-the-shoulder thing – it’s quite like a climbing body harness.

The thing Ripley is wearing? I think it’s just called a “strap on” TBH
You could try typing that in your search?

Tactical Alice Belt/Vest

eBay

same page as ‘powerloader .instructables’ -hot-things-to-do-with-old-bike-tube/

– reminds me of that old rugby song about knitting.

Even better power loader costume