In the original story, and the 1960s adaptation, a desiccated professor, long estranged from his own subconscious, ill-advisedly blows on a lonely whistle that he finds on the beach, and invokes a malign spirit which pursues him.
In the new adaptation the professor has a wife, apparently with Alzheimer's, whom he has left in a care home. He goes to an hotel they visited when young, and finds a wedding ring on the beach. The malign figure which then torments him appears to be the spirit of his wife. Why? Not only why is she doing it, but why complexify a perfectly good stark story with some relationship mumbo-jumbo?
Why muddy the cold waters? What was supposed to be happening? At a guess I would say that John Hurt's devotion to his wife, which was constantly emphasised, was trapping her in this world, dead but unable to die. He had to release her, but he couldn't. Her living ghost was tormenting him with increasing desperation.
The production was good, and John Hurt - so great to see him on telly again - was fantastic. Wonderful in himself. But my feeling is that if they wanted to impose a completely new story, they should have called it a new name.
There is a longer review, well worth reading, on Paperknife here by brisingamen.
ETA - also see this excellent compilation on Paperknife which includes links to the original text, the 1960s TV adaptation, and Robert Powell reading the story.
ETA - Today's Guardian editorial on MR James