Heat milk and water over low heat until lukewarm. Remove from heat, sprinkle with yeast and sugar; let stand 10 minutes.
Sift flour and salt together; stir into yeast mixture. Allow the batter to rise in warm place until bubbly and double in bulk, about an hour.
Mix baking soda with hot water; stir into batter. Allow to rise in a warm place until double in bulk again, about 45 minutes or so.
Heat griddle to medium hot; place 12 muffin rings on top. Spoon equal amounts of butter into each ring; cook until crumpets are golden brown on the bottom. Turn, brown for a minute or two on other side.
To serve, split crumpets, toast and spread thickly with butter and jam.