Preheat oven to 180°C. Combine the oats, flour and 75g sugar in the bowl of a food processor and pulse until it has the texture of breadcrumbs. Add the butter, then pulse several times. Add the syrup, mayonnaise one egg and yolk, then pulse several times, until a dough forms. Wrap and refrigerate for 1 hour.
Peel the apples and cut into wedges. Mix with the lemon juice and remaining sugar in a medium saucepan then set over a moderate heat and cook gently for 15 minutes. Stone the peaches and cut into wedges, then cook for 5 more minutes. Spoon into a 2L pie dish.
Roll the pastry out to a disc 5cm wider than the pie dish, drape over, then fold the edges in and pinch decoratively. Beat the remaining egg and brush over, then scatter with Demerara sugar. Bake for 30-35 minutes, until deep golden. Cool on a wire rack, then serve warm.