This mystified me for a while. It was growing in the woods below the monument.
No interactions present.
... the first time I found it, back in 2008. (It's well established in a pine plantation about a mile away.) I'm now up to about 20 sites, depending on how you count sites. (For example, that are scattered colonies along a wooded ridgeline spread over 3 tetrads and 2 counties.)
The cause of the problem in my case was that I misidentified the family as Fabaceae.
Lat/Lng: 56.1401, -3.9206
OS grid ref: NS807958