Make jelly as per instructions on packet. Leave it on a bench to lower to room temperature. Do not chill as it will set. (I personally use raspberry jelly because of its stronger flavor and you can also taste it over the lemon juice in the filling)

Crush marie biscuits and combine with melted butter. Put in a cake tin and compact it. Put in fridge to cool.

Mix lemon juice (add more or less to taste, i find juice from 2 lemons is ample) and sweetened condensed milk. Add gelatin as per packet instructions. Mix well. Put mixture on top of the biscuit base. Refrigerate until set. Usually 1 to 2 hours is enough. Want to be firm enough to poor the cooled down jelly on top without disturbing the centre layer

Pour jelly on top. Refrigerate untill set. 3+ hours