Preheat the oven to 160°C/fan 140°C/gas mark 3. Gently heat the sugar, golden syrup and butter in a pan until melted.
Add the oats, raisins, walnuts and seeds to the pan until coated in the butter. Spoon half the mixture into a 23 x 23cm baking tin.
Scatter over the plums and nectarines, then top with the remaining oat mixture. Pack the mixture down well and smooth with the back of a metal spoon. Bake in the oven for 35 minutes, or until dark-golden. Leave to cool completely before cutting into 16 bars with a sharp knife.