Complex sentence: Letters sent home by the expedition created the misleading impression that everything was going according to plan, this seems to have been by agreement as certain optimistic phrases kept recurring, but it meant Scotland would be completely unprepared for the coming disaster.
Split into simpler sentences: Letters sent home by the expedition created the misleading impression that everything was going according to plan. This seems to have been by agreement as certain optimistic phrases kept recurring, but it meant the Scottish public would be completely unprepared for the coming disaster.