Preheat the oven to 180 C / Gas mark 4. Grease and flour a 23cm (9 in) square tin. Combine the flour, baking powder, salt, cinnamon and orange zest. Set aside.
In a large bowl, cream together the butter and 150g sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in the walnuts.
Pour batter into prepared tin. Bake in the preheated oven for 40 minutes, or until a skewer inserted into the centre of the cake comes out clean. Allow to cool for 15 minutes, then cut into diamond shapes.
For the Honey Syrup: In a saucepan, combine honey, 200g sugar and water. Bring to a simmer and cook 5 minutes. Stir in lemon juice, bring to the boil and cook for 2 minutes. Pour hot honey syrup over the cooled cake.
Alternatively, you can pour cool honey syrup over the hot cake straight from the oven.