This was very very brave. The production team had run out of money, and had to do an entire story with no guest actors and no sets beyond what had already been made. The two episodes had two different directors, one of whom had never directed a television drama before. It could have been a disaster.
In fact it is very good. I would even have said excellent, were it not for the bathos of the minor technical problem with the Tardis which turns out to be at the core of the plot. But apart from that - and one or two minor slips from Hartnell, though he keeps it together for the big set-piece speeches - I was surprised by just how good it is.
I also watched the DVD documentary, which is entertaining and enlightening, and also actually slightly longer than either of the episodes. Meta-text, isn't that the concept I'm looking for?