I've just come of four EY flight and this is what I learned
If your flight is full then at the check in desk, the checkin agents can do as they please. Be nice to them and they will allocate extra leg room seats. From LHR the guy was nice enougth to select extra leg room on request. He was not willing to do my connecting flight, allbeit I had already preselected. At BKK again the same I was told as it was a full flight she needed to ask her supervisors permission and then allocated. At AUH they seemed to be all trainees on the checkin desk with mentors stood to assist. I insisted on a extra leg room seat without charge and eventually I was allocated it.