It does look lovely, and one for the record books...
"The Long-tailed Blue is an extremely rare migrant to the British Isles. It was first recorded from Brighton in East Sussex, and Christchurch in Hampshire, in August 1859. By 1939 a mere 36 sightings had been recorded - mostly of individuals. Between 1940 and 1988 another 85 sightings were recorded. The only major immigration was in 1945, a good year for migrants in general, when there were 38 sightings. A recent immigrant was observed in Hampshire in 2006."
http://www.ukbutterflies.co.uk/species.php?species=boeticus