In a large bowl combine oats, pastry flour, salt, 1 cup brown sugar and bicarb soda. Mix in the butter until crumbly. Press half of the mixture into the bottom of a 23cm square baking pan.
In a small saucepan over medium heat combine the dates, water and 1/3 cup brown sugar. Bring to a boil and cook until thickened. Stir in lemon juice and remove from heat. Spread the filling over the base and pat the remaining crumb mixture on top.
Bake for 20 to 25 minutes in preheated oven or until top is lightly toasted. Cool before cutting into squares.