When we first meet Rocky, we learn that he's an incredible 85 years old. In fact, Jean observes almost immediately that he can't be Lionel's father because Lionel is 90.
We've been present when he meets Jean and Judith and Sandy and he has completely charmed each of them within seconds because he is spontaneous and witty and kind and sentimental and he can't spare the time to invest in growing old and cynical. For Lionel to turn out the way that he does, he surely must have inherited nine-tenths of his personality from his mother. He doesn't resemble Rocky at all.
Instead of being delighted at the news that his father intends to remarry, Lionel wanders off and pouts. Instead of enjoying the uniqueness of this wonderful house in the country with all its museum-like artifacts and gadgets, he figures that all of this memorabilia adds strength to the argument that his father is "crackers."
We should all be as peculiar as Rocky. A lot of people settle for dreary, redundant lives. They don't learn how to do anything that's new and in a fast-changing society such as the one we have, that's the kiss of death. How would you feel if the doctor who treated you when you were a child didn't bother to learn new techniques and continued to treat you using out-of-date methods? Personal computers first started to appear on the market in the early 1980s. JFK probably never imagined the possibilities. Things that happen every day are happening because of people just like Rocky -- people who are adventurous and peculiar and willing to try new things. Rocky hasn't invented a cure for cancer or discovered a computer chip that will change society, but he IS willing to try new things, and that's why I find him irresistible.
Madge says that the two of them never bother to read warning signs because if they did, they would have missed doing many of the things they've done. Those things include shooting the rapids, wandering the globe, dressing in comfortable clothes, avoiding people they don't want to be around, and getting off on country music. He gives up the security of the big home in Hampshire and opts instead for an unencumbered life with a woman who is a perfect match for him. And, by doing that, he paves the way for Lionel to marry Jean. It's clear that he loves his son very much.
Rocky doesn't appear to be afraid of anything. I that a person who is dying must have suspicions that suggest something is afoot. Rocky doesn't ever let these suspicions get in his way. Clearly Madge knows that Rocky's time is limited. In fact, she says "This won't be the longest marriage on record, but 'by golly' it's tremendous fun." Fortunately for us, Bob Larbey seems to have forgotten this thread because Mr. Hardcastle senior appears to be healthier than a horse.
Rocky is, for me, one of the most likeable characters in the series. I've never seen him waste even a moment of time worrying about his creaking bones or his hearing loss or his lack of success. He's a happy man and it shows.