I am still waiting for a definitive decision on whether a flight that is aborted (where no alternative a/c is available) or delayed more than 3 hours (because of the need to wait for a spare a/c) because the plane has gone tech after the journey has commenced, are or are not extraordinary circumstances.

I have to say that the answer is not obvious. Perhaps it depends on the reason why the plane went tech - poor maintenance could be on one side of the line and a punctured tyre because of debris on the runway on the other ??