Thanks Fi.
I read elsewhere that the heads can be harvested when the backs of the heads turn yellow. I lopped off a couple and kept some of the seeds and put the heads into a better place in the garden for the birds. I might save some of the smaller ones for midwinter when the birds will get better use of them.
We have one plant that went to 9' in the end, another with a head that must have been 18" across in flower and various others. I kept some seeds from the best. I can pass on a few if anyone is interested, though the plants were grown from a mixed packet bought from Poundstretcher.