Line a 12 hole muffin tin with muffin cases and pre-heat the oven to 180°C/ 160°C fan assisted / gas mark 4 / 350°F.
Pour the mincemeat, sour cream, sugar, melted butter, eggs into a large mixing bowl and beat together until combined.
In another large mixing bowl sift together the flour, mixed spice, bicarbonate of soda, baking powder and salt in a mixing bowl, then tip into the liquid batter.
Beat together for a minute or so until everything is well mixed then allow to rest whilst you prepare the streusel topping.
Prepare the streusel by placing the flours, sugar, butter, oats and spice in a large mixing bowl and rubbing together between your fingers. The streusel should come together but still be crumbly.
Give the muffin batter another stir then evenly distribute between the muffin cases.
Crumble the streusel on top of the muffins then bake in the oven for 30 minutes.
Remove from the oven and immediately remove the muffins from the tin and leave on a rack to cool.
Prepare the orange icing by placing the icing sugar into a small mixing bowl and zesting ½ the orange into it.
Juice the orange then whisk 1-2 tablespoons of the orange juice into the icing sugar until the glaze is smooth and pourable.
Drizzle the icing over each muffin and allow to cool before serving.