Arenaria montana Mountain Sandwort
This looked to me and others in our group on a Naturetrek holiday like a Linum (Flax) species and the nearest in appearance was Linum suffruticosum (White Flax). But we had limited identification resources to hand in Spain. Now with main library at hand the ten protruding anthers from the flowers and prostrate habit of the plant indicate a totally different genus and species: Arenaria montana.
The English name of this plant could cause confusion as in the Britsh Isles, Minuartia rubella is also known as Mountain Sandwort.
Roadside rocks outside Miraflores, Sierra Guadarrama, Spain 25th April 2014
Added on 15th May 2014