All plants will seed given half a chance by the way they are biennial not annual. Part of the brassica family plant grows one year and flowers the next - like oil seed rape.

With a bit of luck they will. I usually keep supposedly biennial wallflowers and most flower for another year or two. I regard them as short-lived perennials.

Thank you for your help. They're still flowering well at the moment, and I hope they will for a few years to come.