Dunstan adopts his new name at the end of the second part of this novel, as part of the conversation where he and Diana recognise that they do not have a future together. Diana suggests changing it to Dunstan because "St. Dunstan was a marvellous person and very much like you--mad about learning..." Dunstan is therefore a name that the narrator adopts because, in part at least, St. Dunstan resembles the narrator's character. Diana adds that St. Dunstan successfully resisted temptation in the form of women, and given that the narrator has just managed to reject Diana, even though she wanted to marry him, there is a certain aptness in this choice of name. Note how Dunstan responds to his new name:
I liked the idea of a new name; it suggested new freedom and a new personality. So Diana got some of her father's port and poured it on my head and renamed me.
Dunstan therefore changed his name because it represented a new start and a chance to remake himself and to forget what he had been. It also is important to note that Dunstan chooses to name himself after a saint. This foreshadows his later interest in hagiography. The seriousness with which he takes this name change is highlighted in the mock baptism ceremony that Diana performs. Dunstan is a new person from Dunstable, and the name change represents a new start and a new beginning.