Add the milk to the flour and beat vigorously with a spoon until the mixture is quite stretchy and smooth, about 3-5 minutes.

Gradually mix all the water into the crumpet batter, then cover the bowl with plastic wrap or a tea towel and let the mixture sit for 20 minutes.  Bubbles should form on the surface of the batter.

While the batter is resting, lightly butter four 3 ½-inch (9 cm) crumpet rings and pull out a 10-inch cast iron skillet.

Heat the cast iron skillet over medium heat heat for a minute or two, then turn down the heat to medium low.  Lightly brush the skillet with olive oil.

Place a crumpet ring in one corner of the skillet and spread 3-4 tablespoons of batter inside the ring, filling it about halfway.  If the batter runs out from under the ring, add a little more flour to the batter, but if the batter doesn't form holes after a couple minutes of cooking, add a little more water to the batter.  Once the batter is a good consistency, repeat with the other three crumpet rings.

Cook the crumpets until the edges are set and holes have formed in the top (about 7-8 minutes), then gently remove the crumpet ring with a pair of tongs.  The underside of the crumpet should be a rich golden brown.

Flip over the crumpets and cook briefly on the other side until it's a nice golden brown, about 1-2 minutes.  Place the cooked crumpets on a cooling rack and cover with a tea towel to keep them warm.

Lightly grease the pan again, then continue to cook the crumpets in batches of four until all the batter is gone.  Serve the crumpets immediately with butter and jam, or allow to cool and toast later.