Make the crumble by mixing the flour, oats, butter, sugar, bicarbonate of soda and salt in a food processor until just combined, or rub in by hand.
Tip half of the crumble into the baking tin (a square deep baking tin 20cm square, lined so it comes up two sides for easy removal of the bake once cooked) then press down firmly and
Make the filling by mixing the condensed milk with the lemon zest and lemon juice until thickened. Spread the filling over the baked base. Carefully sprinkle over the remaining crumble and press down lightly.