Gently warm the milk. In a bowl, dissolve the yeast and sugar in the warm milk and 100ml of tepid water. Leave in a warm place for 15 minutes, or until frothy. Sift the flour, bicarbonate of soda and 1 teaspoon of fine sea salt into a large bowl. Make a well in the centre and pour in the yeast mixture. Whisk from the centre outwards until the consistency of double cream – this will take a few minutes. Add a splash more water, if needed. Cover with a damp tea towel and set aside for 45 minutes or until little bubbles form on the surface. For the butter, pour the cream into a freestanding mixer. Whisk on high for 5 to 10 minutes, or until the cream splits into solids and liquid. Drain through a sieve, discard the liquid, then rinse the solids thoroughly with cold water. Place in muslin and squeeze out any excess liquid. Mix ½ a teaspoon of sea salt through and wrap in greaseproof paper. Grease four 10cm egg rings with vegetable oil. Wipe a little oil around a large frying pan or iron skillet and place over a medium-high heat. Arrange the rings in the frying pan and, once hot, spoon 4 tablespoons of the batter into each ring. Cook for 5 minutes, or until little bubbles appear on the surface. Once the bubbles have burst, leaving little holes, use tongs to carefully lift off the rings and flip over the crumpets, then cook for 1 minute on the other side. Re-grease and reheat the rings and pan before cooking the next batch. Serve the crumpets warm, with a slather of butter. If making them in advance, just toast lightly on both sides when you come to serve them.