• These episodes were directed by Frank Cox rather than Marvin Pinfield
• The tunnel shots reminder the OM of The Web of Fear
• The Ood  & Sensorites were neighbours, The Ood Sphere being in the same star system
• Heaven Sent & Hell Bent affirmed this episodes description of Gallifrey
• Joseph Conrad wrote the deeply problematic Heart of Darkness
• The TARDIS scanner has varied significantly across the years

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