Pre-heat the oven to 180°C/ 160°C fan / gas mark 4 and line and grease a 9 inch x 5 inch loaf tin.
First make the almond paste by placing the almonds in a food processor and blitzing until finely ground. Add the caster sugar, golden syrup, vanilla extract and egg white and blitz again until a paste has formed. Set aside for a moment.
Place the basil leaves, lemon zest and orange zest in a food mixer along with the sugar. Whisk for a few minutes together until fragrant.
Add the butter, one cube at a time and beat on high until light and fluffy.
Scrape the almond paste into the creamed butter and sugar and beat until combined.
Beat in the eggs, one at a time.
In a separate bowl whisk together the flour, baking powder and salt then beat it into the rest of the batter.
Roll the blueberries around in the empty flour bowl so that they are gently coated with the remaining dusting of flour then stir into the batter.
Pour into the cake tin, smoothing the top then bake for 60 minutes or until an inserted toothpick comes out clean.
To make the drizzle place the blueberries in a small saucepan with a dash of water and heat for about a couple of minutes until the blueberries are just beginning to soften but not so that they have burst. Leave to cool.
Pour the blueberries into a medium sized bowl along with the lemon juice, basil leaves and granulated sugar. Mix together.
Remove the cake from the tin onto a wire rack, making sure there is a baking tray underneath to catch the excess drizzle.
Prick the cake all over with a cocktail stick and then spoon the drizzle over the cake.
Leave to cool completely before serving.