Combine the flour, salt, sugar and yeast in a mixing bowl and make a well in the centre. Gently warm the milk and water together in a saucepan then pour into the well and whisk until you have a smooth, thick batter. Cover with a damp tea towel and leave the batter for 1 hour in a warm place.
After 1 hour, stir the batter to knock out any air and transfer to a jug.
Heat a nonstick frying pan over a medium heat until hot. Add a little oil and wipe with a piece of kitchen paper to grease the whole pan. Place a crumpet ring in the centre and pour enough batter into it so that it comes about halfway up the sides. Reduce the heat and cook for 6-8 minutes until the surface is blistered all over with bubbles.
Remove the ring and turn the crumpet over, letting is cook for a further 1-2 minutes.
Repeat this process until you have used all the batter, making sure you wipe the pan clean after each crumpet and re-grease the pan with vegetable oil in between making each one.
Leave the crumpets to cool on a wire rack after each one is made.
When ready to eat, toast the crumpets for 2-3 minutes in a toaster. Spread with knobs of butter and serve immediately.