Whisk together the warmed milk and yeast until combined, then whisk in the egg yolks until combined. Sift 350g of the flour into a large mixing bowl. Stir the egg and milk mixture into the flour until combined. Bring together into a very soft dough using your hands, then allow to rest in the bowl for 10 minutes.
Add the butter, sugar, salt and remaining flour to the dough and incorporate well. Turn out onto a floured surface and knead for 4-5 minutes until even. Towards the end of kneading, add the sultanas and knead into the dough. Transfer the dough to a lightly oiled bowl and cover with a damp tea towel. Allow to prove for an hour or until doubled in size.
Butter a 2lb loaf tin with the butter for greasing. Once the dough has risen and doubled in size, knock it back and stretch into an oblong shape. Fold one edge into the centre, then the opposite edge over that edge so you have a rectangle. Press into the tin with the seam-side facing the base. Cover with a damp tea towel and allow to prove again until risen and doubled in size; usually an hour.
Preheat the oven to 200°C.
Bake for 30-35 minutes until golden, and risen, removing halfway through to make a shallow incision in the top of the loaf. Remove and brush the top with the warmed apricot jam to glaze. Transfer to a wire rack to cool completely. Slice and serve on a chopping board lined with a napkin.