Centranthus ruber. Known as Jupiter's beard and kiss me quick. Whilst not a native wildflower it has a wild habit of spreading, not uncontrollably, and finding interesting growing places. The seeds are wind born and it is the favourite food plant of the hummingbird hawk moth, which is a good reason to grow this just to see one of these amazing moths in flight. Grows well in nocks and crannies in lime rich walls and dry banks.