Is it OK to have a redundant path to earth?
Go ahead and put the second earth electrode in

Since your neutral is isolated from earth at the flat, you can safely put an earth electrode at the flat and wire it to the earthing terminal bar at the flat's distribution board.  (This works irrespective of whether your distribution system is TN-C-S, TN-S, or TT.)

There are no concerns about ground loops here since the only currents that flow on earth wires are fault currents, and the ground itself is too poor of a conductor to carry those fault currents.