|Corsican Hellebore, Helleborus argutifolius, at Hayes Station, Kent. 26 March 2013.|
At first I thought it was a Green Hellebore, Helleborus viridis, a native species which is often found as a garden escape. But Green Hellebore's flowers don't have that whitish colour on the outer surface, and while the leaves are toothed, they don't have the spiky look that these do.
There is nothing helpful in Francis Rose's "The Wild Flower Key," which is a standard field guide, but that doesn't include everything you might find. For that you have to turn to Stace's "New Flora of the British Isles," 3rd edition, which includes many hybrids and garden escapes. That's where I found the Corsican Hellebore, with a matching description. "Introduced (...) Grown in gardens, rarely naturalised in marginal habitats."
But there might be other cultivated varieties not even in Stace. There is one in my garden that I once confidently identified as H. niger which I now think is actually a horticultural variety. So I checked photos on line - reasonably convincing - and put a photo on iSpot. I got some agreements straight away, and I am now happy that H. argutifolius is a correct identification.
In this position it can't have been planted on purpose. It's just one stem, so it will be interesting to see if it survives and spreads. I don't think it can really be counted as "naturalised" unless it does.