Had another look at next door's wind poppies (Stylomecon heterophylla) this morning. I first photographed them on 1st June. There are several in flower now and plenty more on the way.
Although native to North America, I don't think they are American (or Californian) Poppies, though they seem to come in the same colour, as those have a more complex leaf structure. It would seem that the wind poppy (aka Blood Drop or Copper Queen) can be bought as seed here and grown successfully. The bee here seems to find it satisfactory, anyhow.