First, to get some things clear: the boy is Mesmer's stepson and the marriage is propably a business arrangement, as hinted by both Mesmer and his wife. As marriages were in those days.
The movie then... Yes, at times Rickman's voice was very hard to hear and the plot seems a bit loose. I admit he's got the most expressive face, eyes, voice and hands in the movie business and for once they were very well portrayed. In fact, the movie seems to rely on them. I was a bit confused, too, what was the point of the movie. Was it a historical piece, romantic story - it even had some comedy, as great tragedies often have. Mesmer seems to fail, although in real life he actually did cure people. And then he said his bit about how much pain there is in the world and how he could not bare not to be able to do anything to relieve it. And there was the point; that's why Dennis Potter, already dying, wrote the script this way. Even the kissing scene, which to many people seems very romantic and sexy, seemed sad to me. As if Mesmer couldn't believe something like that really happens to him. Even the ending made sense, then. This is a man, whose heart is aching to do something good in a world filled with pain and cruelty, selfishness and ignorance, but fails because one man isn't enough. Even the one he cures isn't saved, because the world doesn't understand what he was trying to do, what he was trying to make people see. Dennis Potter's testament, maybe?
Oh yes, Rickman was born to wear cloaks (see Snape and Sheriff of Nottingham) and white ruffles around his throat, Amanda Ooms is lovely and the acting and the sceneries are great. And to remind, why Rickman is so great as a villain, he gets to throw the boy down the stairs. Suddenly a burst of energy and passion, making the character even more complicated, more human - because that's what Rickman does at his best: complete, complicated, deeply human characters.
It's a very different kind of movie and many may not like it, but it's well worth watching. One gets to see an actor like Rickman far too little, and Dennis Potter was an excellent writer, even if this wasn't his best script. I loved it. It was very different from the super hero and cartoon character movies. We need to be reminded of real humanity and the fragility of people this way, because there seems to be very little room for compassion, pity and understanding in today's world.