ROAST DUCK WITH BLUSH ORANGE AND RHUBARB

1.25kg Gressingham Roast in the Bag Whole Duck • 1 tsp Chinese five spice • 1 tsp fresh thyme leaves, plus extra sprigs to serve • 5 garlic cloves, crushed in their skins • 2 blush oranges, juice of 1 (4 tbsp), 1 cut into wedges • 150ml fresh chicken stock • 1 tbsp dark soy sauce • 200g rhubarb, cut into short lengths • 2 tbsp light brown muscovado sugar