Betony is a classic example of the labiate (or mint) family; multiple tubular flowers on a square stem and with pointed, serrated leaves coming out in opposite pairs up the stem. It is also available in white! It is strange how a few white specimens will appear amongst many purple ones (this is espcially true on Corfe Common).
Although betony flowers from June until September it seems to be at its best in August. Betony is at home in woods, on grassland and even heaths and does not mind whether the the soil is calcareous (therefore alkaline) or acid. It is quite common in Dorset where it occurs but it does not occur everywhere, you have to look for it!