Add all purpose flour, salt and baking powder to a batter jug or small bowl and gently combine.
Now add the sourdough starter and water to the flour mixture and whisk until it's the consistency of heavy whipping cream (you might have to add a little extra water if it's too thick).
Heat up a frying pan to a moderate heat (you don't want it too hot or you'll burn the crumpets. You want low and slow).
Brush the inside of the crumpet rings with butter and place them in the frying pan. Scoop the sourdough crumpet mixture into the crumpet rings so that they are around half full.
Place the lid on the frying pan and allow the crumpets to cook low and slow for around 10 minutes. You'll see bubbles start to form on top of the crumpets. Depending on how low your frying pan is, the crumpets might take from 10 to 20 minutes to cook through. If bubbles aren't popping on the surface, gently prick the top of the crumpets with a toothpick to encourage holes to form.
Once bubbles appear on the top of the crumpets and they are cooked through, flip them over to brown the top. If you flip them over before the crumpet has cooked through, you will lose the holes as the uncooked batter will cover them up.
Once both sides of the crumpets are brown, remove them from the pan. Serve warm with butter and jam.