It's interesting that we have such a wealth of high-quality behind-the-scenes material for this story while, at the same time, it's the first one that is completely missing. Possibly a sign of how the BBC's view of the programme shifted from thinking of it as an educational balance between science and history - and so deciding to show off their historical sets and costumes - to viewing it as sci-fi entertainment for families and so seeing no value in archiving the historical stories. Though given how few episode were explicitly preserved (rather than just lucky to survive) I'm possibly reaching a bit to suggest the historicals were any more neglected than the others.