A country-wide study recently found a species of bat not previously thought to be present in Britain: Myotis alcathoe. They are very small ("the size of the end of a thumb") and are more commonly found in Greece and mainland Europe. They are migratory, but scientists thought they couldn't make it over the English Channel-- turns out they can. They had not been noticed before now because they look very similar to some of the other 16 species of bat found in England, which is not very surprising because it's hard to see what bats look like anyway.
Full article: http://news.bbc.co.uk/2/hi/uk_news/england/8632121.stm