I planted a pack of 70 seeds a few years ago and every single seed sprouted. I gave quite a few plants away and all the ones I kept have sprouted into massive bushes.
It's just at the right time of the year to take semi-ripe wood cuttings. 3-4" in pots of compost with a layer of grit.
Place in a cold frame or put polythene bags over the pots.
Yes Tilly, I think it would be best to remove the flower heads, as you want action to take place at the root end, energy channeled into the flowers will be wasted.