Obviously, I can't say on here where the site is, but with great riparain and woodland habitats at hand, plus several large attic spaces, it was a promising site. The first attic space I entered revealed many accumulations of bat droppings, mostly running in a line below the ridge. This was because the bats had been roosting in the space between the fibreboard lining and the roof tiles. Droppings had fallen into the attic through gaps in the fibreboard and these gaps were particularly prevalent at the ridge. Given the small size of the droppings, the large quantitiesof them and the fact that the bats clearly have an affinity for crevices, makes it likely that they are one of the Pipistrelle species, but I can't be certain of that until they return to the roost later in the year. (An advantage of being in Scotland, with a limited range of bat species: I would be far less willing to make even tentative suggestions as to species if I were in the south-east, where there are more than twice as many species!)
Incidentally, you can tell bat droppings from mouse droppings because, although they are a similar tiny size and cylindrical shape, bat droppings tend to be much darker and have a "knobbly" texture because they are largely made up of pieces of hard chitin from insect ectoskeletons. If you rub a bat dropping between your fingers it will usually turn into a gritty dust, whereas the mouse dropping will probably squash between your fingers (yum!). In the pictures you can see how some of the older droppings are turning grey.
The second roost was in a separate attic space. The droppings there, instead of being concentrated in piles, were scattered everywhere. They were also a little larger than the others and had a slightly shiny appearance. It's possible these were left by Brown Long-eared Bats (Plecotus auritus), which tend to roost in the apex of an open attic space and fly around to warm up before emerging from the roost. Later I analysed some of these droppings under the microscope and the shiny appearance was caused by large quantities of moth scales. (Excuse the picture quality below - photomicroscopy isn't my strong point!) Moths form a significant part of BLE food, especially in Scotland, but again, I won't know what species they are for certain until they return later in the year.
If you fancy trying bat dropping analysis (it really isn't as horrid as it sounds) try to get hold of a copy of "Identification of Arthropod Fragments in Bat Droppings" by Caroline Shiel et al, published in 1997 by the Mammal Society. (Try Pennine Books http://www.penninebooks.co.uk/)
My website: http://plecotus.co.uk/