Size is a guess, from memory perhaps 15 mm long.
Distinctive colouring: body a dull olive-green; legs, a shiny deep red plum
The group of ants was fairly static: lots of waving antennae., but all stying in their huddle.
Nearby, other ant, not part of the huddle, but exploring and being busy.
No hint of aggression or defensiveness: when I stepped close to get a better look, I trod on a twig or leaf that went into the middle of the huddle and moved them: but no reaction, they ignored this disturbance.
Thanks JaneR, nice photos & notes. There is some additional information about this ant at the site below. Cheers, Jon https://www.antwiki.org/wiki/Dolichoderus_doriae