Sad news indeed. William Russell played Ian Chesterton from the original 'Pilot Episode' until episode 6 of 'The Chase'. As one of Susan's teachers, he was one of the first of the Doctor's companions to appear in the show
He had originally been intended to reprise his role of for the 1983 story 'Mawdryn Undead', but scheduling conflicts required the story to be rewritten to bring back Nicholas Courtney as another, later regular character, the Brigadier. This caused no end of problems with regards to dating the UNIT stories !
He did return to the programme in 1999 for the VHS release the story 'The Crusade', where he provided linking narration which covered gaps in the narrative due to the episodes being missing.
He provided audio commentaries for various 'Doctor Who' DVD releases, read audio book versions of three early Doctor Who novelisations and provided linking narration for CD releases of the fan-recorded soundtracks of serials with missing episodes in which he appeared
From 2009, he reprised the role of Ian for Big Finish Productions' 'Companion Chronicles' and 'The Lost Stories'. In 2013 he voiced the First Doctor for the anniversary story 'The Light at the End'. At 88 years old at the time of recording, he became the oldest actor to play the Doctor in any medium. He has since reprised the role in subsequent First Doctor releases for The Early Adventures range.
In 2022, he reprised the role of Ian on television for the first time since 1965 in the BBC Centenary Special 'The Power of the Doctor'. In doing so, he became the oldest actor to work on modern televised 'Doctor Who' at the time of their involvement in production, aged 96 at the time his scenes were recorded. His cameo in the episode reportedly broke the Guinness World Record for the longest gap between televised appearances as the same character