tucumseh:
While this was the original opinion of MoD, the Directorate of Air Staffs belatedly admitted in 2005, in writing, that the aircraft may not have been under control, and that the lack of control may only have become evident as they tried to execute the post-waypoint turn (i.e. at the end of a straight leg).
BBF:
Surely if it could climb, it could also have turned.
While it is true that the Chinook Mk2 at that time was cursed with both Engine Control problems (DECU connectors that intermittently didn't and FADEC code riddled with errors) and Flight Control problems (control pallet detachment and loose articles (control springs) causing control jams in one or more axis), I doubt that even the most zealous "airworthiness cause" posters postulate that all of these effects coincided to cause the accident. As you say, Barbie, the aircraft could climb, but apparently it did not/could not turn.
As tucumseh reminds us, the man who could and wanted to give evidence of having experienced these various "surprises" of the Mk2, Sqn Ldr Burke, was not called upon to do so by the BoI. Fortunately for us the FAI and the HoL were more receptive, and his evidence is available there for all to see.