I offer the following from the Staffordshire Bat Group website:

"All British bats are protected by the 1981 Wildlife and Countryside Act. This means that bats and their roosts are protected from harm and interference.

This protection is needed because Bats are in decline. Old records of bat numbers show us that bat populations have decreased markedly. For instance, the numbers of the Greater Horseshoe bat have declined by over 90% in the last 100 years. In addition to this, in the year that Staffordshire Bat Group was formed, the Mouse-Eared bat was declared extinct in the UK!

The reasons for this decline are loss of suitable roosts and feeding sites."

I understand that intensive agriculture practices have also added to the decline by reducing the numbers of suitable insects.