Complex sentence: The role of the Doctor had been played by William Hartnell at the programme's inception in 1963 but by 1966, it was increasingly apparent that Hartnell's health was deteriorating and he was becoming more difficult to work with.
Split into simpler sentences: The role of the Doctor had been played by William Hartnell at the programme's inception in 1963. However, by 1966, it was increasingly apparent that Hartnell's health was deteriorating and he was becoming more difficult to work with.