Here is another small flower I found on the shores of a reservoir near Madrid. Image 7596 was my first sighting of it and shows the needle shaped succulent leaves, 5 pale green sepals, 5 pink petals, 10 stamens with yellow anthers and prominent yellow ovary. Image 7711 shows the general tendril structure of the plant and (as a bonus) has some shoreweed (or slapwort) in the same picture which gives a good impression of the size. I estimate the flower at 7-8 mm diameter.
this is a Spergularia or Sea-spurrey....so named because most species grow at the coast and in rather saline areas. There are few species that occur inland on sandy but not salty environments and only about 3 or 4 that occur in these inland habitats in Spain. We can rule out S. purpurea as that has petals which are significantly longer than the sepals. I'm pretty sure that your plant is Spergularia rubra which is also the commonest species in the UK.
Incidentally was having a senior moment when I called Corrigiola - shoreweed, I meant Strapwort. Shoreweed is Littorella which looks completely different!