Preheat oven to 170 C / Gas 3. Grease and flour one 26cm tube cake tin.
Combine oil, butter and sugar together. Beat well with a mixer. Add eggs, flour, bicarbonate of soda, salt and vanilla; mix well. Stir in apples, coconut, raisins and walnuts. Pour batter in prepared tin.
Bake for 90 minutes. Remove cake from oven. Let it cool in the tin for a few minutes and then remove from tin.
In a small saucepan, heat dark brown soft sugar, butter and milk. Bring mixture to the boil and stir for 1 minute. Pour over warm cake.