For a much more interesting, creative, scientific and downright weird handling of the topic than you have any right to expect here, go to San Diego Natural History Museum: The Truth About the Speckled Band by Laurence M. Klauber.
Holmes diagnosis of the snake leaves a problem; no-one else knows what a Swamp Adder is. One must assume that it was a local and shortlived name that Holmes knew and has since fallen into disuse. As to the snake that Holmes was referring, the most dangerous poisonous snake in India, after the cobra, is the krait. The Banded Krait (Bungarus fasciatus) has all the distinctive characteristics of the krait, but in addition it has across the back large broad bands, their colour alternating yellow and black. The Banded Krait has a very beautiful appearance. Its venom is estimated to be sixteen times as powerful as that of the Cobra. In this species of snake, perhaps with an especially intricate pattern, we surely have our speckled band. (Rosenblatt)
Some sources criticize the snakes ability to climb on a 'dangling rope' but they fail to recognize the significant emphasis on the fact that the bell-pull was both thick and lying on the pillow. More troubling is the fact that snakes do not have ears, yet Roylott trained it to return at the sound of a low whistle. However, snakes can feel vibrations, especially in the low tones. As for the controversy as to whether snakes like milk or not, that is largely moot: the fact is that they will drink it and any kind of food versus no food is never much of a brain-buster, even for a hot-tempered snake.