A bit of detective work and help from my friend Mr Google tells me that my mystery plant is a Phacelia tanacetifolia
, aka Purple Tansy aka the Honey Plant because bees love it so much. As do hover flies, lacewings, and ladybirds. It originated in Mexico and the southwestern USA.
This is one of the plants in my garden - as you can see, the leaves are toothed. When the flower bud appears it's globular and covered in small spikes but as the plant grows (it gets to about two/three feet high) that round 'bud' turns out to be a mass of tightly curled sprays of flowers. As the flowers develop, the sprays uncurl. As you can see, each head of flowers contains several sprays and each plant has more than one head of flowers.
This is what happens when you use a bag of mixed seeds - who knows what magic will grow!