Sebastian wonders offhand about a quote in this episode. He's thinking of St. Augustine, who wrote "Oh, Master, make me chaste and celibate - but not yet!" No doubt Waugh has in mind that Sebastian had come across the quote from Augustine's "Confessions" in his catechetical studies.
This episode is the second time that Anthony Andrews and Jeremy Irons have been depicted making an aristocratic "Grand Tour"-style visit to Venice. In the 1974 adaptation of Anthony Trollope's "The Pallisers", Andrews as Silverbridge Palliser and Irons as his best friend Frank Tregear visit Venice, but in the 19th century.