Preheat the oven to 180'C / Gas 4 and grease and line a 2lb loaf tin.
Add the butter and sugar to a large mixing bowl and beat with an electric whisk until pale and smooth. Then add the eggs, buttermilk and marmalade and mix again for 1 minute. Don't worry if it separates - the flour will fix this!
Next sift in the flour, baking powder and xanthan gum and beat again with the electric mixer until fully combined and forms a smooth batter. Pour into the baking tin and bake in the centre of the oven for one hour to one hour and 10 minutes. It's done when a skewer comes out of the centre clean.
Remove from the oven and place on a cooling rack. Spread the marmalade over the hot cake and leave to cool completely. Once cooled, make the icing by mixing the icing sugar and orange juice to form a thick paste. Pour over the cake and leave to set for 10-15 minutes before slicing and serving.
Notes
Will keep for 3-4 days in an airtight container. This cake can also be frozen before icing.