Mystery flower

Mystery flower

Observation - Mystery flower. Description: Can anyone identify this? I've seen it in three sites, twice at the base of a wall beside a pavement, where it only reaches about 8 inches in height, and once at the local allotments, where it made two large clumps about 18 inches high. Each site is a m

Can anyone identify this? I've seen it in three sites, twice at the base of a wall beside a pavement, where it only reaches about 8 inches in height, and once at the local allotments, where it made two large clumps about 18 inches high. Each site is a mile or two away from the others. It's been around throughout August and September. I've tried 2 wild flower books (Keble Martin and McClintock & Fitter) a knowledgeable relative, a retired gardener, my gardening books in case it's recently escaped, and three websites. After answering questions about colour, size, leaf shape etc. the three all said "No Match"! It's slightly hairy, the leaves are opposite, the centre of the flower is like the centre of a daisy, and the four petals are well separated from each other, and have two notches at the tip. The flowers are small - about half a centimetre in diameter.