The title of the episode is from "The Allegory of Love: A Study in Medieval Tradition", a book by C.S. Lewis. It is an influential exploration of the allegorical treatment of love in the Middle Ages and the Renaissance.
The line "The heart can and should obey the head" is credited to C. S. Lewis. It's a shortened version of a sentence in his book "The Abolition of Man". The exact quote is "The heart never takes the place of the head: but it can, and should, obey it."
This season is the first one where original uk network ITV had, for no apparent reason, started splitting the usual 2 hour tv movie length episode, into 2x1 hour versions (part 1 and part 2) usually shown on 2 nights in the same week. Yet, when their own other sibling networks such as itv3, started re-running Lewis, they decided to use the alternate usual 2 hour tv movie version. This is why the episode listings in imdb for this period, confusingly list part 1 on one date, part 2 on usually the next day, as well as an entry for the 2 hour version. This season, as well as most of the following seasons, actually only has 3 or 4 x 2 hour episodes, as usual, and not 6 or 8 episodes - unless you are watching the 1 hour part 1 and 2 versions.