I'm sure that many of us are presently watching those aircraft holding over the IOM and Liverpool trying to get to LHR (even the anti-spotters are secretly doing it). The commanders of those aircraft all feel that it is safe to land at LHR. Could somebody please explain why it is safe for these aircraft to land at other airports within the 'ash cloud' but not at LHR? Is the ash thicker at LHR than it is at BRU?