AK - It is all in the report, but to summarise; On a previous sortie the lap straps had been routed through the handle with the result that it had been partially pulled, but not enough to fire the seat. After that sortie the pin had been inserted in a way that DID NOT render the handle safe. Numerous checks failed to reveal the incorrect fitting of the pin and on the day of the accident it only took a "nudge" from the full and free check to cause the seat to fire.