Having checked the BBC Genome database, it would seem Moira presented with Peter Sissons that day. I would very much guess that Peter would have led as he usually did.
He always did, because as he points out in his autobiography, he actually had it written in his contract that he would always be the senior presenter whenever he was on air. He says that Anna Ford once told him that he was the only presenter she would be willing to play second fiddle to, and he didn't have the heart to tell her she was actually contractually obliged to do that anyway.
That said, as he also points out, there wasn't much for the senior presenter to do on a programme like the Six. Other than saying good evening at the start and reading the headlines, in all other aspects they were a completely equal partnership because they would faithfully take it in turns to read a story, with no attempt made to give particular stories to any specific presenter or group similar stories together into a sequence. He says that if a story was inserted into the running order, they would still carry on taking it in turns so they would end up reading each other's planned stories for the rest of the news.
The flying fish fingers era Nine was probably the one with the most equal partnership, because one presenter got to say good evening and read the headlines at the start, but the other presenter would read the headlines and say goodnight at the end. Presumably this came about because when John Humphrys and Julia Somerville started on the Nine they were doing it on their own, and when it went double headed they didn't want either of them to feel like they'd been demoted.