No interactions present.
I can't be sure, but I'd suspect J. acutiflorus - brownish not blackish tepals, and the tepals look to be acuminate.
In the field I use Poland's criterion of the number of septa per 5cm of leaf (5-10 in J. articulatus - 1-2 in J. acutiflorus).
Thanks - I should be back at the site soon so will check the number of septa and confirm if this supports your suspicion.
I was back at the site yesterday (15th October) and checked the number of septa on the leaves of several plants. Most had 5-7 septa per 5 cm of leaf confirming J. articulatus based on Poland's criterion. Interestingly, one plant had c3 septa per 5cm of leaf - an abnormal articulatus? Or possibly some hybrid influence?
Lat/Lng: 51.6298, 0.0205
OS grid ref: TQ399942