Charles affirms Cordelia's guess that modern art is "bosh," strengthening it with the superlative "great bosh." The bit of dialogue is a straight quote from the book, illustrating Granada Television's ideal of bringing the heart of the novel to the living room screen. According to David Cliffe's excellent online companion to Brideshead Revisited, "EW [Evelyn Waugh] was something of an artist himself, mainly in drawing and design. (He did six line drawings for his first novel Decline and Fall.) He did however come early to think that modern art was on a wrong course. Picasso was a particular bête noire for him."
The ongoing euphemistic term for Sebastian's malady is "dipsomania," which was how people referred to alcoholism decades ago. The somewhat unlikely word is borrowed from ancient Greek and literally means, "insanely thirsty."
The character of the obnoxious "Rex Mottram" is closely based on a real person, Brendan Bracken (1901-58), an Irish journalist who was a protege of the newspaper magnate Lord Beaverbrook and, through Beaverbrook, a close friend of Winston Churchill's. He pursued a political career as a result, and became Minister of Information during the Second World War, and eventually a Viscount. Evelyn Waugh had a profound contempt for Bracken, whom he regarded as an untalented opportunist, and made no secret of having caricatured him in his book.
The book from which Lady Marchmain is reading after dinner is George Grossmith's "Diary of a Nobody."