Caz
"When the crew selected the waypoint change they would have been approx 600 metres from the cliffs".
While I try not to comment on aircrew matters, and gladly defer to experts on in that area, can you say what possible effect the acknowledged errors (not to mention defects) in the Nav system may have had on this "approximation"? Waypoint selection was via SuperTANS. In many respects it is just a computer - rubbish in, rubbish out. One source of its data is known to be unreliable over water and another had design defects. In another post someone (correctly I believe) suggested that, given the distance flown over water, this "approx 600m" could be more than double.
I'm too old to have repaired TANS but I have extensive knowledge of repairing source systems, for example various Doppler, GMC and Rad Alt. They are all inherently error prone, and the error build-up is, I believe, one reason why the TANS algorithm is weighted in favour of one source over another. I'm being deliberately simplistic on a quite complex technical matter, but my point is that the "approximation" is just that - no-one knows where the waypoint change took place. Therefore, I think using that approximation as a reference point for determining negligence is very tenuous indeed.
My argument is one that no-one knows if the Nav System was telling the crew their correct position. The subsequent Racal "testing" of the surviving bits of TANS can only have taken place in a benign, wholly unrepresentative environment, devoid of typical external influences such as input errors, EMC and poor bonding. Importantly (in my opinion) it is a known fact that the crew were unhappy about TANS (but what they probably meant was they did not trust the indicated result of calculations based on error-prone external data). The MoD's position is that, based on this unrepresentative testing, the Nav System was both accurate and serviceable. (Two quite different things, each dependant on many external factors which were destroyed with ZD576 or are impossible to replicate).
As ever, I am not suggesting this as the cause, but think it constitutes reasonable doubt. I'm quite willing to be corrected on any of this as I'm not overly familiar with the actual Mk2 nav fit.