Combine the yeast, sugar and milk in a small heat-proof bowl or glass jug. Cover the bowl and leave to stand at room temperature for about 10 minutes or until the yeast mixture is frothy. 

Sift the flour and the spices into a large bowl or the bowl of a stand mixer. Rub in the butter either using the mixing attachment or use your fingers if not using a mixer. 

Tip in the frothy yeast mixture and add the sourdough starter, egg and sultanas then mix using the dough hook on low speed until the dough comes together and is soft and no longer sticky. This will take at least 5 minutes. If the dough is still really sticky add a little more flour a couple of tablespoons at a time. 

Transfer the dough into a greased bowl, cover and leave the dough to rise in a warm place until doubled in size. 

Prepare a large baking tin - line it with parchment paper so it extends over the sides. 

Turn the dough out onto a lightly floured surface and knead gently for a minute or two then divide into 16 equal pieces. 

Shape each piece into a ball and pinch it underneath to make the surface smooth. 

Place the dough balls in the prepared pan so they are just touching. 

Cover and leave to rise until almost doubled again (about 20 minutes) and pre-heat the oven to 200C fan-forced. 

Make the flour paste for the crosses by combining the flour and sugar in a cup then gradually mixing in enough of the warm water to make a smooth firm paste. 

Use a piping bag to pipe crosses onto the buns. 

Bake for about 20 minutes or until golden brown on top. 

Make the glaze by heating the water, sugar and gelatine in a saucepan over medium heat.

Transfer to a wire rack, cross side up and still attached to one another. 

Brush the hot buns with the glaze and leave to cool on the rack. 

Serve while warm or leave to cool completely and store in an airtight container or freeze.