Add 2 of the satsuma oranges, the strawberries and raspberries to a large punch bowl or pitcher. Juice the remaining 4 satsumas – you want about 1/2 or 2/3 cup of orange juice. Add the champagne, grand marnier, club soda and brandy to the bowl and stir everything together. Taste the sangria and if it’s not sweet enough for you, add some simple syrup (equal parts sugar and water, boiled then cooled to room temperature) until it reaches the desired sweetness. Serve with a garnish of fresh mint.