Mix crushed biscuits with melted butter. Press firmly into a lined slab tin. Place in the fridge until cold and set.
In a medium bowl, Dissolve gelatin in boiling water and whisk in milk, juice and vanilla. Pour onto base and allow to cool until fairly firm.
Make Jelly according to the packet and allow to cool. Pour over a spoon onto the custard layer. Place in the fridge and allow to set for 4 hours to overnight. Slice and serve.