The details about the horses and the ride in Rotten Row... there's a bit in Murder Must Advertise where Lord Peter needs to establish an alibi, and his friend Charles Parker of Scotland Yard suggests that he might be run away with in Rotten Row and fall off... Lord Peter says something along the lines of: "I don't mind being run away with, and only saving myself through consummate horsemanship, but I draw the line at falling off." In the end, he was seen riding with an attractive young woman connected to the royal family... ANYWAY, clearly I've spent too long semi-memorizing those books, but I thought the whole scenario seemed to echo the one in Murder Must Advertise, and might be a nod to Dorothy L. Sayers as noted Sherlockian. Probably I'm overthinking this!