Mix crushed biscuits with melted butter until evenly coated. Press firmly into the base of a 30 x 18 cm dish in a compact, even layer. Refrigerate while you make the filling.
Dissolve the gelatine in ¾ cup boiling water, stirring until no lumps remain. Add the condensed milk and lemon juice and stir until smooth. Pour over the biscuit base and refrigerate for 45 minutes to 1 hour until firm to the touch.
Once the condensed milk layer is firm, pour the jelly slowly over the back of a large spoon onto the surface, moving across the dish evenly — this prevents it from breaking through the layer beneath. Refrigerate for a minimum of 4 hours or overnight.
