Preheat the oven to 200 degrees C. Grease a 20x30cm baking dish.
In a medium bowl, cream together the butter and brown sugar until smooth. Combine the flour, oats, bicarb soda and cinnamon; stir into the creamed mixture. Mix in the walnuts. Set aside 2 cups of the mixture. Press remaining mixture evenly into the bottom of the prepared pan. Spread jam over the base and crumble the reserved oat mixture over the top.
Bake in the preheated oven for 25 to 30 minutes until golden. Cool before cutting into squares.