So here it is.
I managed to make a list of 24 species, which I whittled down to 12 without too much difficulty. Rather than going with the conventional Top 10 you're going to get a Top 12 - you lucky, lucky people. As always, if I were to produce this list next month (or, to be honest, tomorrow), it would most probably change. So, for the first post, and in descending order:
12. Henbane (Hyoscyamus niger)
This species was, in my world at least, a myth for an awful long while. I just never came across it. The feeling seemed to be that it pops up out of the blue, stays but briefly, and disappears just as quickly. Plus it isn't common in Surrey. There was also 'something of the night' about it, not just the species ghostingly brief appearances, but its veined petals, apparently foul smell and, last but not least, its poisonousness. Possibly a plant that came from an evil or alien place...
Since then I have not clapped eyes on another. A return visit to the same field the following year revealed lots of neat grass but no Henbane. Its mystery is still intact.