BOAC
Why they were there? Perhaps this is "Why".
Almost everybody (except WK) has assumed that the Lighthouse was the planned turning point (because the structure could be seen from a considerable distance in clear weather) and had been misplotted - something highlighted by the BOI.
The initial planning for the sortie was apparently initiated by Lt K and completed by Flt Lt Tapper. Years ago, in the early days of this PPrune saga, it was stated that Flt Lt Tapper had landed several times at the HLS on the Mull which - if true - would suggest that there was perhaps a "standard flight log" for Aldergrove -Mull and return. If so, it is therefore possible that an outbound standard routing for a Chinook to the Mull HLS was inserted into the initial part of the planning for the outbound sortie to Inverness via the Mull - saves a lot of work. This would have brought the aircraft much closer to the high ground than if they had used the Lighthouse as a turning point.
I cannot offer any proof - but it could explain the "Misplot" and why the Crew did not turn immediately when they switched waypoints.
Remember the Flight Safety slogan - "Don't assume - CHECK".