Spiced sweet potato & walnut cake

Name: Spiced sweet potato & walnut cake
Cuisine: European, American
Ingredients: 300g Evangeline sweet potatoes (about 2 small); 135ml rapeseed oil, plus extra for greasing; 135g plain flour; 1 tsp baking powder; ¼ tsp bicarbonate of soda; ½ tsp fine salt; 165g caster sugar; ¾ tsp ground cinnamon; ¾ tsp ground nutmeg; ¾ tsp ground ginger; 2 large eggs; 100g walnuts, toasted and roughly chopped; ICING; 200g soft cheese; 2 tbsp caster sugar; 1 unwaxed lime, zest; Handful walnuts, toasted and roughly chopped