Hairless, with long narrow leaves, small white flowers and small round fruits. Several creeping stems were growing from the same point.
No interactions present.
Crucifers have alternate, not opposite, foliage.
That explains why I couldn't find it in the book!
Isle of Skye
... closeups of the flowers and fruits (and on the other side a photo of the whole plant) might help identification.
One would think that a prostrate plant with linear leaves and tetramerous flowers would be easy to identify, but I've been through a whole book without success.
My first thought, after excluding crucifers, was one of the tetramerous Caryophyllaceae, but I failed to find a match.
Until the identification as Veronica scutellata (I don't know that plant) was offered this was looking like a confusing mixture of plant parts. After the identification was offered everything "clicked into focus" and I could see the structure of the plant.
I've never seen this either, which is strange as according to books and NBN it's really rather common. Not too much in the way of acid habitats round here, though.
... but there are a few sites where I might have seen it, but haven't. (I've seen quite a few plants that are rarer.)
Lat/Lng: 57.23, -5.93
OS grid ref: NG629223