Line and grease a 9 inch loaf tin (2lb loaf tin) and pre-heat oven to 180°C / 160°C fan assisted oven / gas mark 4.
Mix the caster sugar with the lime zest until fragrant.
Beat the butter into the sugar for about 5 minutes until light and creamy, using either a stand mixer or an electric beater. Scrape down the sides of the bowl every so often.
Make the almond paste by whizzing up the almonds in a food processor with the caster sugar until they resemble fine breadcrumbs. Add the golden syrup, vanilla extract, lime juice and egg white and whizz again until well combined and a thick paste has formed.
Add the almond paste to the creamed butter and sugar and mix together.
Beat in the eggs one at a time.
Whisk together the gluten-free flour, desiccated coconut, baking powder and salt in a separate bowl then add to the rest of the batter. Beat well until fully combined.
Pour into the prepared baking tin and bake for 60 minutes or until an inserted cocktail stick comes out clean.
Remove from the oven and leave to cool slightly in the tin whilst you prepare the drizzle.
Mix the lime juice, desiccated coconut and granulated sugar together.
Turn the cake out of the tin and turn the right way up. Poke holes all over the surface using a cocktail stick, then pour the drizzle over.
Cool completely before serving, slicing a fresh lime to decorate the top of the cake.