" A weed is only a flower in the wrong place". I'll go for Common Centaury or Red Bartsia. I'll let the experts narrow it down. Cheers, Tony.
I believe it is Epilobium tetragonum. Common name Square stemmed willowherb.... aptly named as can be seen in Evil Len's pic. https://www.google.co.uk/search?q=epilobium tetragonum&um=1&hl=en&biw=1091&bih=525&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=AGzkT9OmDsbg8gPskei_Cg
