Mouse-ear Cress

Thanks for the ID Marta. I actually have a field guide of Mediterranean plants, recommended by a user, but this plant is not in it. There are two Arabis species, verna & cypria, but they are purple & mauve. Many others are described but not illustrated and in many cases the actual plants do not look like the drawings.
I think the PN system where we all help with what we know is a great idea. I can spend more time outdoors taking photographs of whatever I see knowing that if I cannot identify it using the 170+ guide books I have on my chosen subjects someone else probably can.
For the last 8 years I've looked out of my window, or over the balcony, at my neighbours array of white flowers wondering what they were and now thanks to your help I know.

Arabidopsis, a very common model organism for plant research that is also very abundant in the Mediterranean side of Spain. And you do not need to buy any field guide Malcom, as I very gladly suggest you the ID ;-)