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.