In a medium bowl add gelatine and ½ cup boiling water and mix well until all gelatine dissolved
Add to this the condensed milk and lemon and mix well.
Pour over the chilled base and set aside in the fridge for 30-40 minutes until completely chilled and setting
Once jelly has cooled but not set pour over the milky layer and pop in the fridge until the jelly has set - I poured the jelly over the back of a spoon so as to not disturb white layer.
Once set - remove from fridge for 20-30 minutes and then run your large sharp knife under hot water - then dry - then slice the edges off the slice first.
Using whole long knife (heating & drying as need be between slices) cut down into the square and apply pressure to cut through biscuit layer. Then slice into squares or smaller pieces.
Keep the slice in an airtight container in the fridge