Combine chopped dates and bicarbonate of soda in a heatproof bowl. Pour over boiling water or strong tea. Stir and leave to soak for 20–30 minutes.
Preheat oven to 180°C / 160°C fan / Gas 4. Grease and line a 2lb loaf tin with baking parchment.
Beat butter and brown sugar together for 3–4 minutes until pale and fluffy. Add vanilla extract.
Add eggs one at a time, beating well after each.
Sift in flour, mixed spice, cinnamon, and salt. Fold gently until just combined.
Add the entire date mixture (including all liquid) and chopped walnuts. Fold until fully incorporated. The batter will be quite loose — this is correct.
Pour into prepared tin. Arrange walnut halves on top and scatter with demerara sugar if using.
Bake for 50–60 minutes until deep golden and a skewer comes out clean. Cover with parchment if browning too fast.
Cool in tin for 10 minutes. Lift out and cool completely on a wire rack before slicing.