Combine the flour, salt, sugar, and instant yeast in a large mixing bowl, whisking them together until evenly distributed. Pour in the warm milk and 3/4 cup of warm water, then whisk vigorously for about 2 minutes until you have a smooth batter with no lumps.

Cover the bowl with a clean kitchen towel and place it in a warm spot in your kitchen for about 1 hour. The batter should become very bubbly and almost double in volume, with a pleasant yeasty aroma.

Dissolve the baking soda in 2 tablespoons of warm water, stirring until completely dissolved. Pour this mixture into your risen batter and stir gently to combine.

Heat your frying pan or griddle over medium-low heat and lightly grease both the pan surface and the inside of your crumpet rings with vegetable oil or butter. Place the greased rings directly onto the heated pan to warm them up.

Ladle or pour about 1/3 cup of batter into each ring, filling them roughly halfway. The batter should sizzle gently when it hits the pan. Cook for 8 to 10 minutes without moving them until the surface is covered with holes and appears almost dry, with no wet batter visible. The bottoms should be golden brown, and the tops should look set but pale.

Carefully lift off the crumpet rings using a kitchen towel or tongs, as they'll be hot. If the crumpets stick, gently run a knife around the inside edge to loosen them.

Flip the crumpets over and cook for just 1 to 2 minutes on the holey side to lightly toast the top. This step is optional but adds a nice texture contrast.

Transfer the cooked crumpets to a plate and cover them with a clean towel to keep them warm and soft while you cook the remaining batter. Re-grease your rings and pan between batches to prevent sticking.