Scatter the cloves of garlic (still in their skins) into a roasting tin. Scatter half of the rosemary and half of the blood orange on top.
Insert the remaining orange and rosemary into the cavity of the chicken and gently slide slices of the butter under the skin, being careful not to pierce it.
Place the chicken breast-side up on top of the garlic in the roasting tin.
Pour over half a bottle of Cava and cover with aluminium foil.
Transfer to the oven and roast for 1½ hours.
After 1½ hours cooking time, carefully remove the foil and, using tongs, turn the chicken upside down and baste with the meaty juices from the tin.
Cook the chicken for a further 20 minutes to brown the underside, before turning the chicken back to the original position.
Baste the chicken again. Turn the oven up to 180C/350F/Gas 4 and cook until the chicken is browned all over - you will know when the chicken is cooked as the juices will run clear when the chicken is pierced with a skewer.
When the chicken is cooked, remove the chicken from the tin and leave to rest in a warm place.
For the gravy, remove the garlic skins, rosemary and blood oranges with a slotted spoon and discard - if any soft garlic clove remains in the skins, carefully slip the soft garlic clove out of the peel and into a clean pan. Pour the cooking juices into the pan along with all the soft garlic cloves.
Add half a bottle of Cava to the pan. Simmer over a medium heat. When the volume begins to diminish, add white wine to taste (up to a bottle - though you may want to drink a glass or two from the bottle) and sprinkle in the tarragon. Leave the gravy to simmer gently until reduced by half. Just before serving, whisk in a knob of butter.
Meanwhile, carve the chicken into generous portions.