Line the bottom of a 20cm cake tin with greaseproof paper. Blitz the biscuits in a food processor until they are finely ground, then tip in the melted butter and process until it’s just mixed. Tip the crumb into the prepared cake tin and press it down with a glass or the back of a spoon so that it’s smooth and compacted. Put the tin into the fridge while you make the filling.
Put 100 millilitres of the double cream into a small saucepan and heat until it just starts to simmer. Switch off the heat and leave for 2 minutes, then add the gelatine and stir until dissolved. Pour the cream into a bowl to cool.
In a large mixing bowl, use a spatula to soften the cream cheese. Sieve in the icing sugar, and add the gin, then beat until smooth. Making sure it’s cool but not set, add the gelatine cream and mix well. In a separate bowl, use an electric hand whisk to beat the double cream until it starts to thicken and leave soft peaks. Scrape the cream into the cream cheese mix and fold together. Tip into the cake tin and use a small spatula to smooth the top over. Put into the fridge for 4 hours or overnight.
To make the glaze, slice the lemon and limes 1-2mm thick. Pour the tonic into a wide frying pan or dutch oven. Add the caster sugar and stir over a low heat until it has dissolved. Add the sliced citrus fruits and let it come to a boil, then simmer for 20 minutes until the fruit has softened. Take it off the heat and use a slotted spoon to remove the fruit and spread it out on a wire rack.
Put 100 millilitres of the citrus syrup into a large jug. Add the remaining 500 millilitres of tonic and the lime cordial, and mix well. Put roughly 100 millilitres of this liquid into a small saucepan and add the powdered gelatine. Heat gently over a low heat, stirring constantly until the gelatine has melted - don’t let it come to a boil. Once the gelatine has dissolved, pour the liquid back into the jug, mix together and leave to cool.
Bring the cheesecake out of the fridge and arrange some of the candied citrus slices on top. Slowly pour in the cooled citrus liquid - pour it onto the citrus slices so that you don’t damage the surface of the cheesecake. Add enough liquid to create a layer of 1.5 centimetres. Use tweezers to add small pieces of gold leaf to the jelly. It will float to the top, so once you’ve added enough, put the cheesecake into the fridge. Leave it for around 10 minutes until the jelly starts to set then use a toothpick to push bits of gold leaf further into the jelly. Leave to set for 2 hours in the fridge.
To serve, run a hot pallet knife around the edge of the cheesecake to loosen it from the edge of the tin then carefully transfer it to a serving plate.