Grease and line the base and sides of a 20cm x 30cm slice pan, extending paper 2cm above the sides of the pan.
Process biscuits in a food processor to fine crumbs. Add butter and cinnamon and process again. Press biscuit mixture into the base of prepared pan. Chill for 30 minutes or until firm.
Combine condensed milk with lemon juice in medium bowl.
Combine boiling water and gelatine in a small jug. Stir until completely dissolved, then stir into condensed milk mixture. Pour over biscuit base and return to the refrigerator for 1 hour or until set.
Meanwhile, prepare jelly according to packet instructions, using 100ml less cold water than directed. Allow to come to room temperature. Pour jelly mixture over set milk filling. Chill for a further 1 hour or until jelly is set.
Carefully remove slice from pan, then slice to serve.