In a large bowl, whisk together the milk, ale, melted butter, sugar and yeast. Add the flour and salt and mix to form a dough. Turn out onto an un-floured surface and knead for about 10 minutes until smooth and elastic. Place into a large, lightly oiled bowl, cover with clingfilm and either place in the fridge to rise overnight, or in a warm place until doubled in size - about 1.5 hours.
If the dough was refrigerated, let it come up to room temperature for about half an hour before proceeding with the next step.
Knock back the risen dough then divide it into eight even pieces (I use a scale for accuracy). Shape each piece into a ball then roll it out into a sausage, about 15cm long. Place them well spaced apart on a lightly greased baking tray then leave to rise, uncovered (but not in a drafty spot) for about 30 minutes, until puffy.
Preheat the oven to 200C/400F/gas mark 6; line two baking trays with greased baking parchment (the buns have a tendency to stick). Bring the 1.5 litres of water to the boil in a large pan, stir in the bicarbonate of soda then turn down the heat until the water is not even simmering.
Poach the risen buns two at a time for 30 seconds per side (I use a stopwatch) then use a fish slice to transfer them to the lined baking trays; repeat with the remaining buns.
Brush the buns with beaten egg then use a lame, extremely sharp knife or razor blade (I use an old fashioned razor blade) to cut three fairly deep diagonal slashes in each bun. Sprinkle the buns with flaky sea salt.
Bake in the preheated oven for about 15-20 minutes, until deep brown; don't take them out too early - a lot of the flavour comes from the dark colour.
Transfer the buns to a wire rack to cool. Eat on the day they are baked.
These really are best eaten freshly baked but can be frozen as soon as they are cool. Allow to defrost at room temperature then refresh in a low oven for 5 minutes or so to serve.