Heat the oven to 160°C, fan 140°C,gas 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 oaty 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 mins or until dark golden. Leave to cool completely before cutting into 16 bars with a sharp knife.