Pipistrelles are thought to have undergone a significant decline in numbers in Britain

Bats are the world's only true flying mammals and one of the most diverse mammal groups on Earth. There are 16 recognised species of bats breeding in the UK. The Pipistrelles are Britain's smallest bats. Pipistrelles are the most common bats in towns and suburbs. Pipistrelles catch their food in flight and usually eat while on the wing. They mainly eat flies such as Midges and Caddis flies. They will also take Mosquitoes, Gnats, Mayflies, Lacewings and occasionally small Moths. Pipistrelles are thought to have undergone a significant decline in numbers. Estimates suggest a population decline of approximately 70% between 1978 and 1993. The increased use of insecticides has reduced the amount of insect prey available to Pipistrelles during their active season.

Source: The University of Hull
http://www2.hull.ac.uk/discover/hbp/actionplan/bats.aspx