Crumpets are made on a griddle and to get their round shape you will need 4-inch (10 cm) tart rings or empty tuna tins with the tops and bottom cut out to use as molds.
The crumpets are best enjoyed warm from the griddle or toasted. The crumpets will keep in an airtight container for up to 2 days or can be frozen for up to 3 months.
Stir the all-purpose and bread flours together with the sugar, yeast and cream of tartar. Add the water and stir with a wooden spoon until blended and smooth – the batter will be dense but sticky. Cover the bowl with plastic wrap and set aside to rise on the counter for an hour. The batter may rise and fall during this time, but that is OK.
Stir the milk, baking soda and salt together in a small dish and add this to the batter all at once, stirring until blended – the batter will be wet but stretchy.
Preheat a griddle over medium heat and grease it well. Grease 3 to 4 (as many as can fit on your griddle) 4-inch (10 cm) tart rings well and place them on the griddle. Spoon enough batter into each ring so that it spreads to fill the ring and is about ½ -inch (25 mm) full. Cook the crumpets for about 5 minutes, loosening the rings part way through, until golden brown on the bottom (and bubbles break through the top of each crumpet), then remove the rings, flip, and cook another 1-2 minutes, but not brown. Remove the crumpets to a plate and continue making until all of the batter has been used.