Torquil's honest but non-aggressive approach to romance obviously appeals to Joan, and it's not long before she realizes that what her heart wants is very different from what her brain tells her she should want. It's apparent when Joan speaks to Sir Robert on the 'phone that she is unconsciously comparing the two men as she asks her fiance what's wrong with his voice. Of course, it's exactly the same as always, but having become accustomed to Torquil's warm tones, she is now vaguely dissatisfied with Sir Robert's voice.
Speaking of Sir Robert, we never actually see him in person... we only hear him on the telephone and hear what other people say about him. What is obvious is that Sir Robert is a snob: when talking to Joan, he tells her to make the acquaintance of the Robinsons because, as he says, there's no one else in the area "worth knowing". Uh huh. Then these so-called 'unworthy' locals, unaware of who Joan is, gossip humourously about how Sir Robert swanks around "like a little king". We can also get a sense of what he's like by observing his best friends, the Robinsons. They are completely uninterested in the area and its history... so it appears the attraction in renting an estate there is merely the prestige of having done so. It seems likely this is also what motivated Sir Robert's residency in the isles. Incidentally, Joan's visit with the Robinsons gives her a glimpse of what life as Sir Robert's wife is going to be like. Having gone to see them in hopes of escaping Torquil's disturbing presence, she is instead presented with a contrast between his warm, easy conversation with friends, and the Robinson's inane prattle. It's not a happy comparison.
* The scenes involving the Corryvreckan Whirlpool were very skillfully done, combining actual footage of Corryvreckan with some from a second location, the Gray Dogs, as well as some studio shots.