In reply to TobyA:
Corvids nesting in cracks is most likely to be jackdaws. Won't be ravens, they build large ramshackle nests that look like they should fall apart (but don't).
Rooks nest communally, usually in a group of trees. Don't know if carrion crows (the normal "crow") ever nest in cracks, but they normally also nest in trees.
Note that young jackdaws in a crack/nest will peck your fingers - anything that moves might be food 😁
Post edited at 23:43