Blink and you could miss them - but mysterious sightings of an extremely rare butterfly have set the hearts of enthusiasts fluttering. The species, previously described as extinct in Britain for nearly 100 years, has suddenly appeared in countryside on the edge of London. Small numbers of black-veined whites have been spotted flying in fields and hedgerows in south-east London. To the non-expert, they could easily be mistaken for the common or garden cabbage white butterflies seen in Britain every summer. But there's nothing common about the black-veined white on this side of the Channel. First listed as a British...