Lightly grease base and sides of a 19cm square cake pan and line with baking paper. Cover base with a single layer of lattice biscuits, placed sugared side down. Place boiling water in a heatproof bowl. Sprinkle with gelatine while stirring briskly with a fork until gelatine dissolves. Set aside for 3 minutes to cool slightly.
Meanwhile, place cream cheese and icing sugar in a medium bowl and beat with an electric mixer until smooth. Add dissolved gelatine, lemon rind and juice and beat until well mixed. Add custard and beat until combined. Spoon mixture into prepared pan and smooth surface. Top with a single layer of lattice biscuits, sugared side up (you will have several biscuits left over). Press biscuits lightly into filling. Cover with plastic wrap and refrigerate for 2 hours or until set. Cut into squares.