Kay
Mortar can fall but it is unlikely to carry on like this, the most likely cause is mice, rats or squirrel, (don't think the Glis Glis has got to your part of the world yet) they may just be in the cavity at the moment hence no sign in the loft space. Rodents knaw, thats what rodent means, and they often chew cables which is commonly the end of them and can cause fires or expensive repairs, problem is there is seldon just the one!! ATB!