In a large bowl, whisk together flour, instant yeast, salt and sugar until the dry ingredients are evenly combined.

Pour the warm water into the dry mixture and whisk until a smooth, thin batter forms; it should resemble thin pancake batter.

Cover the bowl with a tea towel or plastic wrap and let the batter rise at room temperature for 1 hour 15 minutes, or until bubbly and expanded.

Stir the batter, then sprinkle in the baking soda and whisk just until incorporated; the batter will thicken slightly.

Preheat a non-stick griddle or heavy skillet over medium heat for 2–3 minutes; a drop of water should sizzle and evaporate in about 3–4 seconds.

Generously butter the insides of the crumpet rings and the cooking surface, place the rings on the griddle and allow them to heat for 30 seconds.

Using a jug or ladle, pour batter into each ring until it is three-quarters full. Cook undisturbed for 4–5 minutes, until the top looks set and riddled with holes.

Carefully lift off the rings with tongs, flip each crumpet, and cook the second side for 1–2 minutes until pale golden.

Transfer cooked crumpets to a wire rack and repeat with remaining batter, greasing rings lightly between batches.