Three days ago, on 27 April 2016, I had a volunteer shift and afterwards, as the sun was peeping through the clouds, I called in at the Reader Rock Garden. There was a reasonable number of plants in bloom, including the one in my photo. I couldn't find any other photos of this plant in my photostream, but I'm sure I have photographed it before. So far, I haven't been able to find out the name of this plant.

Thanks to Rob & Sharon Illingworth for letting me know the ID of this plant. After checking again the Reader Rock Garden's website, I discovered that this species is growing there. Apparently, this is a quite easy to grow perennial from the Caucasus and Turkey.