Preheat oven to 150 C / Gas 2. Grease and flour a 26cm round tube cake tin. Mix together the flour, bicarbonate of soda, salt, allspice, cinnamon and nutmeg. Set aside.
In a medium bowl, whisk together 300g sugar, oil, eggs, milk and 4 tablespoons rum until blended. Make a well in the centre of the dry ingredients. Pour in the rum mixture, stirring until combined. Stir in the walnuts.
Pour batter into prepared tin. Bake in the preheated oven for 60 minutes or until a skewer inserted into the centre of the cake comes out clean. Allow to cool in the tin. Pierce cake from 15 to 20 times with a wooden skewer to prepare the cake for the rum glaze.
Rum glaze: In a saucepan, melt 225g butter in water, then bring to a slow boil. Stir in 200g sugar and continue to boil for 5 minutes, stirring constantly. Let mixture cool for 5 minutes, then mix in 285ml rum. Let cool 15 minutes, then apply the rum glaze to the cake.
To apply rum glaze: Gradually drizzle a small amount of the rum glaze over the top of the cake while it is still in the tin. When the top of the cake cannot absorb any more glaze, turn it out of the tin and onto a plate to repeat the procedure on the bottom of the cake. Let the cake sit until the glaze has soaked in, then repeat the procedure from the beginning, putting the cake back into the tin to glaze the top. It may take 24 hours for the cake to completely absorb all of the rum glaze.
If you can get your hands on black walnuts, use these instead.