they were meant to be showing the ice warriors in this timeslot but it was presumably removed due to last minute takebacks on episodes that were considered lost and were presumably intended to be released on stream. (they certainly promoted having lost episodes and almost all stories with one missing episode were not shown with the exception of this story and web of fear. + there's a lot of ice warriors footage in the trailers.) this is just my personal conspiracy as to why it wasn't shown anyway.

not that i'm bitter- got a recording of this instead, which is a story i do love dearly