Thanks to a thread at missing episodes, I’ve discovered there were actually two Dr Who omnibuses prior to Christmas 1976 – Pyramids of Mars on 27th November and The Brain of Morbius on 3rd December, both one hour long and presumably scheduled to fill in the odd mid-season gap between The Deadly Assassin Part 4 on 20th November and The Face of Evil Part 1 on 1st Jan 1977.
An omnibus Seeds of Doom was scheduled to follow the first two repeats on 11th December but was pulled at the last minute in favour of Gerry Anderson’s outer space odyssey Into Infinity. I’ve heard Into Infinity referred to in the same breath as The Seeds of Doom-omnibus-which-never-was but never quite got the sense of it, assuming that Seeds would definitely have been on 27th December as per usual. So I’m grateful to Richard Bignell for the correction.
Quite why the Pyramids and Morbius omnibuses have fallen through my memory I have no idea.