Found this again this year. Few more photos:


Latter shot has a shoot of Yarrow in the background.
I'm still pretty much in favour of
Petrorhagia saxifraga. I've found a few photographs on Flickr which are pretty similar (obviously subject to caveats: I recently came across one labelled Alpine Sow-thistle for a roadside plant in SW Wales), e.g., this
one. Here's a link to the
Flickr search.
The plant is obviously thriving with plenty of flowers. If there are suggestions as to any more detective work I can do, I might be able to collect some material.