The distribution of the Barbastelle Barbastella barbastellus extends from Morocco north to southern Britain and east to the Caucasus. The range is not continuous and the species is absent from much of southern Europe (Stebbings 1988; Stebbings & Griffith 1986). In the last cebtury, major declines occured in all but the extreme east of its range, including in the UK. Suggested reasons for the decline vary from loss of roosts to timber treatment and to the availability of its preferred insect prey. The annual number of records from Britain up to 1995 had been too low and scattered to represent a viable breeding population, and in consequence it had been recognised that a population of perhaps 5,000 individuals had to exist but had remained undiscovered (Harris et al. 1995).