Heat oven to 160C/140C fan/gas 3. Grease and line a deep 20cm round cake tin with baking parchment.
Put the gluten free flour, bicarb and all the spices in a food processor, add the lactose free butter and whizz until it has disappeared, then tip into a mixing bowl (or rub in the lactose free butter with your fingertips until the mixture resembles fine breadcrumbs).
Put the sugar, treacle, golden syrup and lactose free milk in a saucepan and heat, stirring gently, until the sugar has dissolved. Turn up the heat and bring the mixture to just below boiling point.
Pour the treacle mixture into the dry ingredients, stirring with a wooden spoon as you go. Beat in the egg until the mixture resembles a thick pancake batter.
Pour into the prepared tin and bake for 50 mins-1 hr, until a skewer poked in the centre comes out clean. Leave to cool completely in the tin.
Split the cake through the middle. For the drizzle, stir the lemon zest and juice into the sugars, then immediately spoon all over the top half of the cake.
For the filling, mix the lemon curd with the diced ginger. For the frosting, beat together the ginger syrup, lactose free soft cheese, lactose free cream and icing sugar until thick enough to hold its shape.
Spread the lemon curd filling all over the cut side of the bottom cake, dollop the creamy icing on top, then sandwich with the other half of the cake.
The unfilled ginger cake will mature well for up to 3-4 days if wrapped well in baking parchment then foil.