18 January 1941: birth of Christopher H. Bidmead, script editor for Season 18 (the last Fourth Doctor season) and writer of Logopolis (Fourth Doctor, 1981), Castrovalva (Fifth Doctor, 1982) and Terminus (Fifth Doctor, 1984) and the novelisations Happy 80th birthday, sir!
ii) broadcast anniversaries
18 January 1964: broadcast of "The Expedition", fifth episode of the story we now call The Daleks. The Thals agree to attack the Dalek city. Ian and Barbara accompany one group through the jungle and the Lake of Mutations.
18 January 1969: broadcast of fourth episode of The Krotons. The Doctor makes up some sulphuric acid; Zoe uses it to poison off the Krotons, and Jamie uses it to destroy the Dynotrope.
18 January 1975: broadcast of fourth episode of Robot, which is the first I remember watching all the way through. The robot starts disintegrating people, but the Doctor manages to destroy it with metal-eating virus.
18 January 1982: broadcast of first episode of Four to Doomsday (the first Fifth Doctor story filmed, as opposed to shown). Rather than Heathrow, the Tardis lands on a spaceship controlled by Monarch and inhabited by humans from four different eras of history.
18 January 1983: broadcast of first episode of Snakedance. Rather than Earth, the Tardis lands on Manussa, home of the Mara; Tegan is possessed by it and gets away from the Doctor and Nyssa.
Edited to add: 18 January 2010: broadcast of Liberation, second episode of the Australian K-9 series. Starkey goes into to hiding from the Department and the surviving Jixen warrior. Jorjie finds out that the Department has imprisoned innocent aliens (including a Zephon and an Axon), so K9 and the team break into the Department's prison to set them free.