Preheat oven to 180 degrees C. Use a deep sided 25cm tin, or wrap the outside of a 25cm springform pan with aluminium foil to prevent leaking. Sift together the flour, baking powder, salt and cinnamon. Set aside.
In a saucepan over medium heat, combine brown sugar and 130g butter. Bring to a boil then pour into bottom of prepared cake tin. Sprinkle with sliced peaches.
In a large bowl, cream together margarine and the white sugar until light and fluffy. Beat in the eggs one at a time then stir in the vanilla. Beat in the flour mixture alternately with the milk. Pour batter over toffee and fruit in tin.
Bake in the preheated oven for 90 minutes or until a toothpick inserted into the centre of the cake comes out clean. Cool in tin for 10 minutes then invert onto serving platter and carefully remove tin. Be extremely careful of hot toffee and fruit juices! Serve warm.