The BBC will be making a new version of the Arthurian legend, but this time Merlin will be the same age as Arthur Pendragon. Merlin will be shown learning how to control his powers even as he helps Arthur to prepare for kingship.
Among the many actors to be in the production are John Hurt, Richard Wilson, and Michelle Ryan (but not as the 'Bionic Woman').
This tweaking of the established legend will place this new version of Merlin's story in an alternate dimension. I'm not sure which portrayal of Merlin should be considered canon in Toobworld, but I would think that since the mini-series starring Sam Neil would have the inside edge. Its coverage of Merlin's life was extensive and covered much of the material in the legends.
As for other portrayals of Merlin in episodes of TV shows, it could be said that the difference in his appearance from that of Sam Neil could be attributed to magic. If it's the whole legend that's portrayed in TV movies or mini-series, then most likely those versions are in alternate dimensions just like this new one.
When it comes to the many variations of Mark Twain's story about "The Connecticut Yankee In King Arthur's Court", it could be said that each version begins on Earth Prime-Time (including those with Dyan Cannon and Keshia Knight Pulliam), but by traveling back in Time each "Connecticut Yankee" creates an alternate dimension.
And as poor a sitcom as 'Mr. Merlin' was, I see no problem with Barnard Hughes' portrayal of Max Merlin to be a VERY much older Merlin once played by Sam Neil......