No interactions present.
Definitely not Galium saxatile, which has whorls of sharply pointed leaves. How big was it? Could it be a sandwort?
Making out detail of both the flowers and the foliage is hard, but it seems to have apical inflorescences of small white tetramerous flowers and leaves in whorls (e.g. a whorl of 6 below the flowers to the lower left of the photo). I can't think of anything other than Galium that would match.
It certainly doesn't jump out at me as Galium saxatile, but that does have broader leaves that most other species, and does have leaves in whorls of 6. It's also variable in habit, from prostrate to trailing. But the absence of the mucrons is a point against, as is the succulent appearance of the leaves. (Have the mucrons been obscured by foreshortening? We may not have a plan view of any of the leaves.)
I was wondering about whether it was one of the forms of Galium palustre, but I don't find that convincing either.
It's at the limit of visibility, but I think the flowers are tetramerous. Can it be confirmed from the original photographs?
I had thought that the leaves were in opposite pairs, but I can see some in a whorl, as you say. The plant must be absolutely tiny, if that is a lens cap in the second picture.
I think in the absence of a photo which shows the leaf tips and edges clearly, this will remain a mystery!
The plant was photographed 580m up a Torridonian sandstone mountain - that means an inclement climate and an infertile soil - so finding dwarfed specimens is not surprising.
(Cul Mor is the only place I've seen ptarmigans below 2,000 feet.)
Come on, Cothill, please add to this discussion with your views!
Original photo added - best detail I could get - leaves on right-hand side stand out best, flowers lower left show the most (but minimal) detail. It was a lens cap in the second photo - it was a very small plant!
The mucrons are just about visible, but I still can't see the prickles. But I think that it must be Galium saxatile.
Those leaves look so succulent: could it be a sedum?
It reallly doesn't look much like a Galium to me....
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 58.059, -5.107
OS grid ref: NC167119