In reply to drconline: Short answer is it depends both on the species (to a certain extent) and the type of roost. Bats are usually loyal to their roosts year after year if it is a good roost. Especially for maternity roosts (where females gather to have their young) which are found from around May to August, and are very vulnerable to disturbance (females can abandon their young if disturbed badly).
However bats sometimes have a selection of summer roosts that they use which may also include temporary resting roosts too, which they may not return to year after year.
They also often won’t use the same roost year round too. So in late spring/summer, females will gather in maternity roosts while males often roost individually or in small groups else where. Then after the young are grown (around August/September) females will leave and form mating roosts with males. Then around November they will move to cooler, quieter hibernation roosts until the following spring.
So if this crack is a summer roost the bats probably (but not certainly) won’t be there in winter.
Similar to birds nests, bat roosts are protected by law (under the Habitat Regulations) and it is illegal to intentionally disturb them. So if you know they are roosting on a route probably best to avoid until they are gone.