It was a long time ago and none of us were there but:
Failed to mark the waypoints off on the chart. (They MUST have had some sort of chart)
Didn't do a track check out of the last waypoint (Mt Hallet?)
Sent the spare pilot back and kept the spare engineer in the cockpit for the lo-level bit. (Big mistake. Gotta be.)
Relied on the INS which they knew at that stage could have been 15 miles out.
What was the weather radar picture?
Didn't ping on the no VHF, no radar contact, and where is this mountain which is supposed to be fifteen miles away on our left??
Feel free to pick me up on any of that if you feel like it, anyone. !!!!!!!!!!!