Walter:-
as Tuc asks, it would be interesting to know if switching off just cuts off the CDU or stops all processing - but it does not matter.
I am fairly certain that the last 5 words in your quote are yours and not tuc's, because it does matter, very much. I don't query that superTANS was on and in use during the flight, quite possibly right up until WP change. What I do doubt is that it was selected ON at impact, because Mr Cable believes that it wasn't and I believe him.
So the question is why did the crew select it OFF, so soon before crashing? It seems to me highly probable that action was a desperate attempt to resolve a serious malfunction that they suspected, rightly or wrongly, was caused by EMI. They didn't know as much as we do now about the Gross Negligence that had forced this grossly unairworthy aircraft into Squadron Service, but they knew enough to distrust it completely. That switch position was their verdict on the airworthiness of ZD576 and their message to fellow aviators. Typically it was ignored by the BoI, the AOC, the AOC-in-C, and the MOD, but was picked up by the AAIB. If there was ever proof that the RAF is not to be trusted to conduct Military Air Accident Investigations, it is in the position of that little switch in the crash debris.