PPRT,
It isn't thought that he tried to eject. The cartridge life was discovered as a result of any investigation of this type probing into everything. This bulletin is a standard monthly report and there was a section on Hunter crash because there were findings to report. Resources are expended on all aspects of the accident until each thread is either exhausted or concluded. In this case, the AAIB has uncovered something that it feels is relevant to the safe operation of, in this case, ex-military aircraft fitted with ejection seats; an issue that affects aircraft currently being operated. Meanwhile resources are expended on many other threads. Therefore, they continue to report as they find.
The AAIB don't ground jets, the CAA have that authority. AAIB investigate, report facts and, where appropriate, make recommendations.