Although the pipistrelle is the most common bat in Britain, it has been declining over the years along with all other bats. The 18 species of British bat are all protected under the Wildlife and Countryside Act. If any of them have made a home in your roof it is against the law to interfere with them. They do no harm to you or your property - so there is no need to worry if you have resident bats!
The widespread use of pesticides drastically reduces insect populations thus depriving bats of food - they may also have eaten insects poisoned with chemicals. Over the years the habitats e.g. hedges, ponds and old grassland, where bats like to hunt have declined in number and they have also lost many of their traditional roosting places, such as hollow trees.
If you would like to find out more about bats and how to help them, visit the The Bat Conservation Trust.