Combine the milk, warm water, yeast and sugar in a small bowl, whisk well and leave to stand for 10 minutes until foamy.
Add the salt, strong flour and whole wheat flour to the bowl of a stand mixer fitted with a dough hook. Add the water/milk mixture, 1 egg and soft butter and mix on medium-low speed until is combined. Continue to mix on a medium-low speed for 2-3 minutes. If the dough appears too sticky, add a little more flour, 1 tbsp at a time, until it pulls away from the sides and creates a cohesive dough. Continue kneading for 5-6 minutes or until the dough is smooth and elastic. The dough should bounce back when indented with a finger.
Loosely cover the bowl with a clean tea towel and let it rise in a warm place for 1 – 2 hours, until double in size. Alternatively, the dough can be left overnight in the fridge for a slow rise.
Turn the dough out onto a lightly floured surface and cut into 4 equal pieces.
Form each piece of dough into a ball and place on a lined baking sheet, allowing enough room between them for the second rise. Brush with oil, cover loosely with cling film, and allow to rise for about 1 - 2 hour in a warm place, until about doubled in size.
Heat the oven to 180°C fan/354°F/gas 6. Beat the remaining egg with a splash of water and brush the entire exposed surface of the buns. Bake them for 7 minutes, then remove them from the oven, brush with more egg wash and sprinkle with Whitworths Four Seed mix. Place back in the oven and bake for 10-15 minutes longer until a deep, shiny, golden brown, rotating the baking sheet halfway through baking if required. Transfer the buns to a wire rack and allow to cool completely.