BBC Radiophonic Workshop: The Alchemists Of Sound (2003)
The Delian Mode – Delia Derbyshire documentary
How Delia Derbyshire made her music
The enduring magic of the Radiophonic Workshop | Resident Advisor
Music Arcade – Radiophonic Workshop 3
From time 6:22 there’s a bit about creating the TARDIS sound effects. You can also
see a follow-up, below: