Nic's said that he's only mentioning song names of tracks which will actually be in the set. They rehearsed about 40, of which 25 or so are in the final set. He specifically won't name "the most difficult song" but mentioned it was one of the old prog songs. So perhaps we can surmise that they rehearsed something with really complicated drumming (Watcher?) and have elected not to play it.
Songs mentioned include:
Firth Of Fifth
In The Cage
Apocalypse In 9/8
Home By The Sea
Mama
Otherwise he confirms that they were "show ready" after the November 2020 rehearsals and that they'll likely do about two weeks of rehearsals again before the Autumn dates. He's very optimistic that they'll go ahead.
I think I've heard him talk about Apocalypse in three separate interviews, each time the heavy suggestion is that it's in the concert set.