A specimen of one of the 'unusual' plants that I brought to the attention of my companions on my most recent visit to Crayford Marshes (22/9/09 - see separate posting), because I didn't recognise it, was subsequently sent by one of them, Margot Godfrey, to the chap who is writing a paper on rare plants of London. She has rung me to say that he is very excited because he has confirmed that it is Slender Hare's-ear (Bupleurum tenuissimum), a small, very slender umbellifer. Apparently this species has not been recorded this far up the Thames, nor within the London boundary, since the nineteenth century.
In the light of this development, Margot made a subsequent visit and counted over 100 individuals of this annual plant. I can also report (4/11/09 update) that the herbarium specimens collected at this site were held up as the highlight of the plant material exhibited at the London Natural History Society's AGM / Flora of London Project talk which I attended.
A photo of this species, taken in East Sussex, can be found here: