Heat oven to 160C/140C fan/gas 3. Line a 900g/ 2lb loaf tin with greaseproof paper. Place the flour, mixed spice, caster sugar, zests, dried fruit and a pinch of salt into a large bowl.
Melt the butter and 2 tbsp marmalade in a saucepan, then stir in the milk. Pour liquid into the dry ingredients, add the white wine vinegar and mix well.
Pour the cake mixture into the lined loaf tin and bake for 1 hr, or until an inserted skewer comes out clean.
Meanwhile, make the glaze by heating the remaining marmalade with 2 tbsp water and the icing sugar until bubbling and syrupy. When the cake is cooked remove it from the oven, pour over the marmalade glaze, then sprinkle with demerara sugar. Leave to coolin the tin.