Prepare the jelly topping by mixing the jelly crystals with the boiling water and then cold water until crystals are dissolved. Note that we are making up the jelly using 50ml less water than the instructions on the package.
Leave the jelly out so it cools to room temperature.
Grease a slice tin (27 x 17cm) and line base with two layers of baking paper, allowing the baking paper to overhang each side (to assist with removal once it's ready to slice).
Crush the biscuits until they resemble fine breadcrumbs using a rolling pin, chopper or food processor.
Add melted butter to the biscuit crumbs and stir to combine.
Press the biscuit mixture into the bottom of the slice tin and smooth with the back of a metal tablespoon. Place into fridge.
Pour condensed milk into a bowl. Add the lemon juice and use a whisk to combine.
Dissolve gelatine in the boiling water and then add this to the lemon and condensed milk mixture. Use the whisk to combine.
Remove the slice tin from the fridge and pour over the filling mixture. Return to fridge for at least one hour to set.
Once jelly has reached room temperature and filling has been in fridge for at least one hour, remove the slice tin from the fridge and place in a larger dish (this will make it easier to transfer to the fridge and catch any jelly spills).
Pour the jelly over the second layer of the slice. (TIP: Pour onto a metal spoon to break up the flow and avoid the direct flow creating a hole in the filling layer of the slice).
Return to the fridge and allow to set for at least 6 hours.
Remove slice from tin using the overhanging baking paper to ease the whole slab of slice from the tin. (TIP: If this method doesn't work, cut one row in the tin and then use a spatula to remove the remaining slice.) Cut into 24 pieces using a sharp knife.