Preheat the oven to 200°C fan/220°C/gas mark 7.
Take the chicken and, starting at the neck end, gently ease the skin of the chicken away from the breast, taking care not to tear the delicate skin. Using your fingers, spread three-quarters of the paste as evenly as possible under the skin all over the breast and thighs, then gently pat the skin to even out the paste. Next, fill the chicken cavity with the two reserved lemon halves and bunch of mixed herbs.
Line a roasting tin with the carrots and leeks and drizzle with a little extra virgin olive oil. Place the chicken on top of the vegetables – this will prevent the chicken from sticking to the tin. Season the chicken all over with some salt and pepper, drizzle with a little extra virgin olive oil, rubbing well all over. Pour in the wine and water and cover with foil.
Reduce the oven temperature to 180°C fan/200°C/gas mark 6 and roast the chicken for about 1 hour 30-40 minutes, removing the foil for the last 30 minutes, until cooked through. During cooking, baste the chicken with the juices from time to time.
Remove the roasting tin from the oven and immediately place the remaining butter on top of the chicken to melt, using a spatula to spread it all over. Carefully remove the chicken and vegetables, place on a board and leave to rest for about 10 minutes before carving.
In the meantime, make the gravy. Combine the juices left in the roasting tin with the juice of ½ lemon, about 200ml of water and place over a high heat, whisk in the flour and continue mixing until the gravy has thickened slightly. Strain, pour into a jug and serve with the roast chicken and vegetables.