Preheat the oven to 190°C/fan170°C/gas 5. Wash the guinea fowl and dry thoroughly with kitchen paper. Season inside and out. Put 3 rosemary sprigs inside each bird and place in a large roasting tin. Squeeze a lemon half over each bird, then stuff the squeezed half, along with an unsqueezed half and 2 garlic cloves, inside each cavity. Loosely cover the tin with foil.
Roast in the oven for 30 minutes, then remove the foil and roast for a further 20-30 minutes or until golden and cooked through. (To check if they are ready, insert a skewer into the thickest part of the thighs - the juices should run clear.) Transfer to a platter, cover loosely with foil and leave to rest.
Meanwhile, make the gravy. Heat half the butter and the olive oil in a medium saucepan over a low heat. Add the shallots. Cover and cook, stirring occasionally, for 30 minutes or until lightly golden. Remove the lid and cook for a further 15 minutes or until tender and caramelised. Stir in the rosemary and cook briefly, then add the vinegar - stand back, as it may splutter. Simmer vigorously until reduced and syrupy, then add the stock and simmer for a further 10 minutes or until reduced slightly. Mash together the remaining butter and the flour to make a beurre manié. Gradually whisk this into the sauce and simmer until thickened slightly. Season and keep warm.
Transfer the birds to a chopping board and remove the legs. Cut through the joint to separate into thighs and drumsticks, then remove the breasts in 1 piece and cut each into 3 pieces on the diagonal. Divide the meat between 6 plates, pour the gravy over the top and serve immediately.
