In the bowl of a stand mixer, combine the milk, sugar and yeast. Let them sit until the yeast starts to bubble, about 5 minutes.
Add the flour and salt to the yeast mixture and beat on medium-high speed for 3 minutes, or until the batter is smooth and it stretches when you lift out the beater. Remove the bowl from the mixer and cover tightly with plastic wrap. Place in a warm, draft-free area until the mixture has doubled in size and is bubbly on top, about 1 hour.
In a small bowl, stir together the water and baking soda. Stir this mixture into the batter, then set aside for 30 minutes.
Heat a large skillet over medium low heat. Spray the surface lightly with nonstick cooking spray, and spray four 4-inch-diameter crumpet molds.
Place the molds in the skillet. Add 1/4-cup of the batter to the center of each mold and then cook until the bottoms are a deep golden brown and air bubbles have formed on the top, about 7-9 minutes. Use a pair of tongs to remove the molds, then turn the crumpets over to cook on the second side for about a minute.
Remove the crumpets to a plate or serving platter, then repeat until you have used all of the batter.
Serve warm.