Place the dates in a bowl with the butter and bicarbonate of soda. Pour over the boiling water and stir until the butter has melted. Set aside to cool.
Preheat the oven to 180 C / Gas 4. Lightly grease an 18cm round deep cake tin and line the bottom with baking parchment.
Place the sugar and eggs in a large bowl and beat well. Stir in the cooled date mixture, then sift in the flours, baking powder, mixed spice and salt. Add the pecans and stir together until thoroughly mixed. Tip the mixture into the prepared tin and level the top.
Bake in the preheated oven until the cake is risen and nicely browned and a skewer inserted in the centre comes out clean, about 1 1/4 hours.
Turn out onto a wire rack and leave to cool. Using a skewer, poke holes evenly over the top of the cake. Stir together the icing sugar and lemon juice, and drizzle over the cooled cake.
The cake can be kept, wrapped in foil or stored in an airtight container, for up to 5 days.