Preheat the oven to 200C/400F/Gas 6.
For the guinea fowl, smear the guinea fowl all over with the softened butter, then season, to taste, with salt and freshly ground black pepper. Place the seasoned guinea fowl into a deep-sided roasting tray.
Transfer the guinea fowl to the oven and roast for 35-40 minutes, or until the juices run clear when a skewer is inserted into the guinea fowl at the thickest part of the thigh.
When the guinea fowl are cooked, tip the juices from the cavity of each bird back into the roasting tray. Transfer the guinea fowl to a warmed serving platter, cover with aluminium foil and set aside to rest.
For the tarragon and lemon sauce, heat the cooking juices in the roasting tray over a medium heat, scraping any burned bits up from the bottom of the pan using a wooden spoon.
Whisk in the flour until the mixture forms a paste. Cook the paste for 25-30 seconds, whisking continuously, until smooth.
Gradually add the lemon juice and white wine, whisking vigorously until combined. Bring the mixture to the boil, then continue to cook for 1-2 minutes, or until the volume of liquid has reduced and the sauce has thickened.
Strain the sauce through a fine sieve into a small pan and return to a medium heat. Bring the sauce to a gentle simmer, then stir in the chopped tarragon and cream. Continue to cook until heated through, then season, to taste, with salt and freshly ground black pepper.
To serve, carve the guinea fowl into slices and arrange the slices onto a serving platter. Pour over the tarragon and lemon sauce. Serve with buttered rice and boiled baby leeks.