Line the base and the sides of deep 28 x 18cm slice tin with baking paper, making sure you leave plenty hanging over the sides.
Crush the biscuits, either in a food processor or with a rolling pin by hand until they resemble fine breadcrumbs.
Add the melted butter to the crushed biscuits and mix until combined. Pour the mixture into the lined slice tin and press down with the back of a metal spoon until level and place in the fridge to set.
Meanwhile, add 2 1/12 teaspoons of gelatine to ¾ of a cup of boiling water and stir until it has dissolved. Place the condensed milk into a bowl and while whisking, add the dissolved gelatine mixture and the lemon juice. Continue to whisk until mixture is well combined.
Remove the set biscuit base from the fridge and pour over the condensed milk layer. Place back in the fridge, until it just begins to start to set – approximately half an hour.
Make the lime jelly as per the packet instructions – adding an extra teaspoon of gelatine. Allow the jelly to cool until it has reached room temperature.
Remove the slice from the fridge and place it on a baking tray as this makes it much easier to move in and out of the fridge. Carefully pour the lime jelly over the slice and place it back in the fridge to set approximately 3 hours and is should be fine.
Make the strawberry jelly as per the packet instructions and add an extra teaspoon of gelatine. Allow the jelly to cool until it’s room temperature.
Remove slice from the fridge and carefully pour over the lime jelly layer. Place the slice back into the fridge - preferably overnight to set.
Remove the set slice from the fridge and cut into squares.