St. James’s
The site of Marchmain House in St. James’s indicates how prestigious the Marchmains are. It is the very best area of London.

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across the park
We know that Marchmain House is in Saint James’s, which is the prestigious area between Buckingham Palace and Whitehall, and that Charles lives in Bayswater, so the park is almost certainly Hyde Park, which lies between the two, though Charles might, if he were walking, go through Green Park first. No doubt the taxi took the route via Piccadilly and Park Lane.
We know from the first edition of BR that the Ryder house is in Hyde Park Gardens, a small road lying just behind Bayswater Road on the north side of Hyde Park itself.)

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Green Park
one of central London’s major parks. Iain Gale (in his book Waugh’s World) places Marchmain House in Arlington Street next to Green Park. It certainly is in Saint James’s, and so close to Buckingham Palace.

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