Hiya,
Does anyone know if the Customer Joined/Left Segment trigger still fires when the segment rules itself are changed (rather than the customer meeting/not meeting the required parameters).
Thanks in advance
Mm from my tests - it seems the answer is no 
if the Segment itself gets updated and the customer joins/leaves the segment as a result the Flow does not trigger.
Correct. As that points, its a new segment and so what behavior it should take is unclear. I think some people would want it to fire and others would not.