Place the flour and salt into a large bowl and stir in the sugar and yeast making a well in the centre. Pour in the warm milk and water and mix to give quite a thick batter. Beat well until completely combined and cover with a damp 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 and pour into a large jug (or you can ladle it in from the bowl).
Heat a non-stick frying pan over a very low heat with a drop of oil. Wipe the pan with kitchen paper to remove excess oil. Sit the greased crumpet rings in the pan and leave to heat up for a couple of minutes.
Pour in enough mixture to fill the rings just over halfway up the sides. Leave to cook until plenty of small holes appear on the surface and the batter has just dried out. This will take about 8-10 minutes.
Remove the rings and turn over the crumpets to cook for a further minute or two on the other side. Sit the first batch of crumpets in the oven, on a wire rack, to keep warm while continuing to cook the remaining mixture.