Posted: 21/05/2014 at 15:33
Basically you don't do anything much. I'd set some white foxglove seeds and scatter them there now. They're biennials so they'll off this year, disappear over winter, reappear next spring and bloom about now. Then they will scatter their seeds and most of them will die and you pull them up in the autumn/winter.
Next spring I'd scatter some more white seeds about, so you have succession the following year, but after that they'll probably seed themselves about quite happily.
If you've got wild ones about they'll probably cross-pollinate and you'll get some with the pinky mauve hues - you can usually tell those plants as they're beginning to develop flower spikes if not before, as they have pinkish tinges, so if you want to keep to all white ones you can pull the pink ones out.
If you get a lot of cross-pollinated ones you might find it better just to scatter some white seeds every spring.