Caves of Androzani.
It was good then, it's good now.
I just listened to Mission to Magnus a 6/Peri lost story by the same writer as Caves featuring Sil and the Ice Warriors.
Androzani was by Robert Holmes, who did far too many Who stories to mention, starting with The Krotons and ending when he died halfway through writing the end of Trial of a Time Lord. Mission to Magnus was by Phillip Martin, who did the other two Sil stories, and whose crime series Gangsters becomes one of the most bizarrely surreal things ever produced.
Weird, I am sure Phillip Martin said he wrote Androzani in one of Toby Hadokes Who's Round podcasts.
I must have mis-heard him.