To understand why they are so popular with bats, look at the schematic below. In a typical medieval building, arches and ceilings are built from stone blocks arranged into a curve, so that each is supported by the one outside of it. Above this, the infill is usually made of rubble. Over the years the mortar tends to fall out of the gaps between the stone blocks. As long misguided perfectionists don't repoint the stonework, this creates crevices, some of which may extend into spaces within the rubble fill, creating sometimes quite large bat havens. The downside for us is that surveying these buildings quickly causes a sore neck, from peering upwards, pointing a light into the crevices!
What is particularly interesting about the site we visited today is that, not only is it used by hibernating Brown Long-eared Bats (Plecotus auritus), two of which we saw, it is also used by Pipistrelles (Pipistrellus sp.). Remarkably little is known about the hibernating behaviour of Pipistrelle bats: they are rarely found in the mines and caves where larger British bat species are found hibernating. It is generally assumed that they hibernate individually in small crevices in trees or buildings and are probably more tolerant of fluctutations in temperature and humidity. So it was nice to find at least twelve of them here.
Unfortunately, it isn't possible to say whether they were Soprano Pipistrelles (Pipistrellus pygmaeus) or Common Pipistrelles (Pipistrellus pipistrellus): the physical distinctions are difficult enough with a bat in the hand! However, a braver bat-worker than I might note the dark muzzles and wonder if they might be commons.
Click on the picture below to see a larger version of it and you'll see the brown splodge in the centre resolve into a Pipistrelle's face.
The following two pictures show two groups of bats, sharing crevices. I have no idea how many are hiding behind the mortar in the first picture, but the crevice in the second contained seven bats.