Preheat the oven to fan 160C/conventional 180C/gas 4. Butter and line an 18cm round, 7cm deep cake tin with greaseproof or parchment paper.
Put the flour, bicarbonate of soda and all the spices into a large mixing bowl. Add the butter and rub it into the flour with your fingertips until the mixture resembles fine breadcrumbs.
Put the sugar, treacle, syrup and milk in a medium saucepan and heat, gently stirring until the sugar has dissolved. Turn up the heat and bring the mixture to just below boiling point.
Add the stem ginger to the flour mixture, then pour in the treacle mixture, stirring as you go with a wooden spoon. Break in the egg and beat until all the mixture is combined and it resembles a thick pancake batter. Pour this into prepared tin and bake for 50 minutes-1 hour, until a skewer pushed into the centre of the cake comes out fairly clean. Leave to cool completely in tin before turning cake out. (To freeze: wrap in greaseproof paper, then in cling film. Freeze for up to 1 month.)
To make the icing, mix together icing sugar and lemon zest, then gradually add lemon juice until you have a smooth, slightly runny icing, adding more juice, if needed. Drizzle icing in a zig-zag pattern over surface of cake, turn cake around and drizzle again to create the cross-hatched finish (see below). Cake keeps for up to 2 weeks stored in an airtight container.