You ask whom you should contact and it should be a Chartered Building Surveyor (not a builder). The Surveyor will diagnose the problem, suggest the remedy and will prepare a claim under your insurance (including recovery of his fee). It sounds to me as if your Managing Agent is trying to brush you off. Your reference to a "gap at the bottom of the wall" puzzles me, and if you could post a photograph of it here I might be able to get you going in the right direction.