Grease a 17 x 23cm cake tin and line with parchment. Heat oven to 180C/160C fan/gas 4. Melt the butter, sugar and syrup together in a large pan. While you wait, stone and quarter the plums.
When the pan ingredients are smooth, stir in the flour, baking powder, ¼ tsp salt, the gingers, oats and eggs, then beat until even. Pour all but about 2 tbsp of the batter into the prepared tin, then scatter the batter with the plums.
For the topping, work the extra flour and oats into the reserved 2 tbsp of batter to make a flapjack-like dough. Crumble this over the tin and scatter with the chopped ginger. Bake for 30 mins or until golden and risen, then cool completely in the tin. Cut into squares.