One walk goes in the morning and one in the afternoon, and both meet at the Christmas tree in Trafalgar Square. At 11am, you can take a walk titled Christmas Morning, 1660 - Samuel Pepys's London, and then later at 2pm a Christmas Day Charles Dickens's London Walk will undoubtedly evoke that Christmas Carol spirit which is usually reserved only for Hollywood.
Your author takes his hat off to them. He is flying out on holiday this evening but this site will to be updated, hopefully every day. For more on the walks, see here.