If you leave them to seed n their own they should do the job for you. Ours do anyway. My problem is oyulling out all the ones in the wrong place...

If you cut back some of them to under where the flowers were you should get another flush but leave some to self seed and you will be rewarded with new plants next year. Like Steragram I do have to weed out quite a few.

Thank you both very much.
I now know what to do - very much appreciated.

Yes if you leave then they will freely seed each year giving you a succession of flowers, that said they are biennial which means they will seed and form a rosette of growth the first year and flower the following, and then die off, but sometimes they will form extra growth at the base which can be seen at the back end when you cut them back, and these will repeat flower for you.

I agree with the others and I usually leave mine to self seed then yank out the ones in the wrong place. However, it doesn't seem to matter what colour I plant the new seedlings have always reverted to the wild pink colour. Whether it's something I do or not I don't know but those I planted were white.