Growing on top of a weed barrier, in shallow soil beneath a layer of small stones.
No interactions present.
It's first flower appeared about 2 weeks ago. Today it has as many as you see in the photo with some buds yet to open.
Actually, Dianthus is also likely. Willowherb is not because they have 4 petals. Another possibility is Saponaria ocymoides (not sure of the spelling). Sorry I can't help any further, good luck with it!
Yes true, I did see that they had 4 petals when I looked at the picture, silly me. The reason I put up Dianthus as well as Phlox is because the phlox pictures have petals that overlap rather than these gappy ones.
Looking at a write up of Saponaria and I dont think that can be it. It describes a seed pod only 10mm long. These flowers are born singly on the end of a seed pod which is around 25mm long, much larger than the flower itself.
Lat/Lng: 51.8, -2.2
OS grid ref: SO8819