Fans of Peter Jackson's Lord of the Rings trilogy who have also read the original JRR Tolkien books will no doubt remember the uncomfortable moment in The Return of the King when Hugo Weaving's Elrond rocks up rather randomly at Dunharrow and starts spouting nonsense about Arwen's fate being tied inextricably to the fate of the ring. It wasn't in the book, and it wouldn't have been in the movie had Jackson and his team not been determined to give the romance between Aragorn and his elven belle more screen time in line with Hollywood convention.

Never mind, most of us thought, forgiving a film-making team that had pulled off a minor miracle in bringing Tolkien's fantasy work to the big screen at all, and managed to do so with great care and brio. I wonder if we'll be so generous if new rumours that Jackson is planning to change the end of The Hobbit turn out to have any basis in fact.

Empire online spoke recently to Benedict Cumberbatch, the Sherlock star suspected of possessing his own magic ring that requires all major Hollywood directors to cast him in their latest productions. As well as voicing the dragon Smaug and the Necromancer (who morphs into evil eye Sauron for the Lord of the Rings) in Jackson's two-part take on The Hobbit, the British actor is also in line for a turn as the major villain in JJ Abrams's Star Trek II and is currently starring as Major Stewart in Steven Spielberg's Oscar-tipped War Horse. If you don't want to know anything about his Hobbity ventures before December 2013, when second instalment There and Back Again hits cinemas, TURN AWAY NOW.

"I'm playing Smaug through motion-capture and voicing the Necromancer, which is a character in the Five Legions War or something which I'm meant to understand," Cumberbatch told Empire. "He's not actually in the original Hobbit. It's something [Peter Jackson]'s taken from Lord of the Rings that he wants to put in there."

Hang on a minute. The Necromancer at the Battle of Five Armies (which is surely what Cumberbatch is referring to here)? Tolkienistas will know that the aforementioned conflict, a five-way rumpus involving dwarves, elves, goblins, wargs and men for the treasures of Erebor, marks the denouement of The Hobbit. There is little indication in the book that it has anything much to do with Sauron, who has recently been kicked out of Mirkwood by the White Council in events we hear about from Gandalf in retrospect. The idea that the Necromancer turns up to lead a battalion of (presumably) goblins seems to come from way out of left field. It's a bit like remaking the original Star Wars trilogy and inserting the ewoks in the first movie (OK, perhaps not quite that bad).

Could Cumberbatch have got it wrong? He does seem a little confused about his Tolkien terminology, so we can only hope that befuddlement is to blame here. With so many different roles to play in 2012 and beyond, who can blame him for getting a little mixed up?

Jackson has already shown a propensity towards presenting The Hobbit in a form which allows it to segue comfortably into the Lord of the Rings, and there's not much wrong with that. After all, Tolkien himself revised his earlier book after delving into deeper, darker territory in its three-part sequel. Few have complained that Galadriel, Saruman and even Legolas are due to appear in The Hobbit, since the book's background events offer Jackson some licence to include them, but allowing the Necromancer to play a major role in the film project's finale seems to me a step too far.

The Hobbit's major villain is Smaug, and nothing should be allowed to undermine that. The mean old worm may be an evil brute with a heart of frozen, inky darkness, but he is unconnected to Sauron and the dark lord's more ambitious machinations. Most would be happy to see the new films prefigure the rise of evil in Middle Earth, but few would expect to see the later books' villain promoted to a major antagonist before his time. By all means let's see the Necromancer get kicked out of Mirkwood, but please keep him well clear of the Lonely Mountain. Mordor is, after all, rather a long way south.