> Must admit though, it seems increasingly likely to have been the aeroplane breaking up.
No, not really. The message (as I understand it) indicates that the plane is probably descending faster than normal. With the result that the pressure outside is increasing faster than the pressure inside. That is not what you would expect with a broken cabin, since you then should have about the same pressure on both sides, since they would be connected.
We have no way of knowing if that is a controlled descent or uncontrolled. However, the fact that it was received by the satellite indicates that the antenna was still tracking, and that argues that the plane was reasonably stable at that moment.