To answer Roy, I know it seems improbable, but a drip from two floors up can often be heard in a quiet flat.
At night, I can hear rain dripping onto my outside window sill... even with triple glazing.
Not that we can be sure that this is the answer..........
But, LB, most definitely needs to be investigated by the Management. As with the roof, or the outside walls, this should come under the heading "Fabric of the Building." I say "should" because, in my opinion, although this drainage is internal, it is not actually within your flat. It's between the flat and the outside walls.
IF this is the problem, be prepared for possible argument over this. It may depend on the wording of your lease.
I think it's well worth pursuing with them Ladybirder.