Preheat the oven to 180°C/fan 160°C/gas mark 4. Cut 2 of the rhubarb stalks into 2-3cm pieces and place in a small saucepan with the orange juice and ginger over a low/medium heat. Cook for 5-10 minutes or until completely softened. Put this in a sieve over a bowl to remove excess moisture and leave to stand for 10-15 minutes or until cooled. Reserve the liquid.
Mix the cooled rhubarb with the butter and thyme and season to taste.
Lay the chicken on a baking sheet with a rim and use your fingers to loosen the skin from the breasts. Spread the butter under the skin of the breasts, reserving 1 tbsp. Cut two of the other rhubarb stalks into thirds and place in the cavity of the chicken. Roast for 20 minutes per 500g, plus an additional 20 minutes, basting regularly.
Half an hour before the chicken has finished cooking, cut the remaining 2 stalks of rhubarb into 3cm pieces and scatter around the chicken on the roasting tray. Spread the remaining 1 tbsp of rhubarb butter over the chicken and pour the remaining cooking juice from the stewed rhubarb over the top. Return to the oven for the remaining 30 minutes.
Once the chicken has finished cooking, set it aside to rest for 15-20 minutes, covered. In a jug, make up 500ml chicken stock using the stock cube.
Remove the rhubarb from the roasting tin and set aside. Add the flour to the roasting tin and stir to combine it with the pan juices. Place over a medium heat and cook, stirring continuously, for 2 minutes, then add the stock very gradually to prevent lumps from forming. Use a sauce whisk to beat out any lumps. Once all the stock has been added, stir in the rhubarb from the roasting tray and the red wine vinegar and cook for a further 3 minutes until thickened. Serve with the roast chicken.