From the Radio Times:

5.30 Colour
Dr Who
starring Tom Baker in
The Ark in Space
A four-part story by
ROBERT HOLMES
Part 3
Humankind faces final destruction.  Can the Doctor defeat the Wirrn?
Incidental music by DUDLEY SIMPSON
Designer ROGER MURRAY-LEACH
Producer PHILIP HINCHCLIFFE
Director RODNEY BENNETT

From the Radio Times (This is for the omnibus edition that repeated the show in a condensed version without episode breaks):

6.35 Dr Who
Ark in Space
A complete adventure
in one programme starring
Tom Baker as Dr Who
Written by ROBERT HOLMES
Beyond the 30th century the survivors of planet Earth lie in crygenic suspension.  But the Doctor finds there has been a sinister intrusion.
Producer PHILIP HINCHCLIFFE
Director RODNEY BENNETT (Repeat)