Karl Popper was one of the most important liberal thinkers of the 20th century, as much for his development of an evolutionary understanding of human knowledge as for his more overtly political books.
For an excellent short introduction to his work see the book by Bryan Mageee.
In 1988 Popper took part in three programmes for Channel 4 under the title Uncertain Truth. In the first one he was interviewed by Ernst Gombrich, and by John Eccles and then Anthony Quinton in the next two.
Those programmes are on Youtube. Each is split in two, there are six videos.
This is the first.