No credit to me, I picked it up elsewhere.
I recall that the ILS was notamed "unmonitored". This begs the question: why did they fly it? They briefed for the VOR approach in this knowledge but they still flew the ILS. There is no way to check that an ILS is working, other than a check against unconnected navaids, or a GPS.
I suppose the answer is in their company procedures manual but I would not have flown a known-suspect IAP without secondary position monitoring. They did the secondary monitoring (DME) and went missed immediately upon realising it didn't make sense. But they did leave it quite late, IMHO.
It's easy to ask these questions, for us "GA pilots" flying with single autopilots, single ILS receivers and with no redundancy in anything. We know that relying on a single source for anything important (like navigation, and thus terrain clearance) is stupid. When we fly with an autopilot we watch it because we know the thing could pack up at any time and do so without any indication of fault (and they do; don't I know it). Flying an ILS, we probably keep half an eye on a GPS, just to make sure we are heading for the right airport
In fact many of us fly nonprecision approaches on the GPS overlay, and use the navaid receiver as a monitor rather than primary (the only safe way to fly an NDB one, IMHO).
It's easy to forget that airliners often don't have a moving map GPS so the crew has no independent and clear confirmation of where they are. They fly blind, relying on the FMS. The FMS is very good and has multiple redundant inputs, but you can't beat a moving map. I haven't got a clue what % of airliners has no GPS but I bet it is the great majority of the world's fleet. Most of the time, in the civilised world anyway, it doesn't matter because anytime they are near terrain they are under radar control.
I've read quite a number of books on airliner accidents and it's obvious that if the crew had something like a Garmin 296 velcroed to the dash, and glanced at it every once in a while, the accident would not have happened.