Combine the flour and salt in a large bowl, then make a well in the flour mixture and add the sugar and yeast in the well. Pour in the warm milk and water and mix to make quite a thick batter. Beat well until completely combined, then cover with a tea towel or cling film. Leave in a warm place to rise for about an hour until it's a light, spongy texture.
Stir well to knock out any air, add the baking soda and pour into a large jug - mix well and allow to stand for a further 30 minutes.
Preheat oven to 300 degrees F. Grease a muffin pan with oil or butter, then place the pan in the oven to heat up. When the pan is hot and the oven is preheated, take the pan out and pour batter into each hole just a half inch high. Return pan to the oven.
Leave to cook until lots of small holes appear on the surface and the batter has just dried out. This will take about 8-10 minutes.
Take the pan out of the oven and flip the crumpets, then return the pan to the oven for another minute or two.
When they've finished cooking on both sides, turn them out onto a cooling rack and continue with the next batch of crumpets.
Serve the crumpets warm, generously buttered. If you are making crumpets in advance, then reheat them by toasting lightly on both sides before serving.