Preheat the oven to 180°C.
Place the guinea fowl into a roasting tray. Put a small cube of butter and 2 sprigs of tarragon into the cavity of the fowl. Rub the skin all over with the remaining butter, then cover the chest with the bacon.
Bake for 1 hour and 15 minutes while regularly basting with the juices. Remove the bacon for the last 15 minutes of roasting.
When the skin is golden, remove it from the oven and cover with foil.
Pour the wine and sherry into the baking tray, then add the stock. Bring to the boil, return the guinea fowl (whole or cut into four portions) and add the grapes. Season with salt & pepper, drizzle with lemon juice. Cover with foil and roast for a further 20 minutes.
Serve with the gravy, grapes and a couple of side dishes.