Preheat oven to 200°C (180°C for fan assisted ovens)/Gas Mark 6.
Unroll pastry onto a floured work surface and use a 20cm plate as a template. Cut away the excess pastry so you are left with a circular pastry sheet. Carefully transfer the pastry using the rolling pin to a lightly greased baking tray and gently prick the pastry. Place in the fridge to chill while you prepare the peaches.
Cut the peaches in half, de-stone and slice them. Put the peaches in a bowl and toss with the caster sugar and cinnamon.
Remove the pastry from the fridge and arrange the peaches in a rosette pattern. Brush with melted butter and sprinkle with 25g of the Demerara sugar. Bake in the oven for approximately 20-25 minutes.
Whilst the tart is still warm glaze with warm apricot jam and sprinkle with the remaining Demerara sugar.