Preheat the oven to 180°C (gas mark 4), conventional setting. Butter a 22–24 cm round cake tin and dust with flour.
In a large bowl, whisk together the pumpkin purée, caster sugar, sunflower oil and eggs until smooth.
In another bowl, sift together the plain flour, baking powder, ground cinnamon, ground nutmeg and salt.
Gradually fold the dry ingredients into the wet mixture using a spatula until just combined and no lumps remain.
Pour the batter into the prepared tin and bake for 40–50 minutes, or until a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 10 minutes, then turn out onto a wire rack and cool completely.