Dissolve the caster sugar and water together, bring to the boil until slightly reduced and cool. Add the orange juice.
Whisk the syrup into the mascarpone. Sieve and churn in an ice cream machine. If making by hand put into a freezer-proof box and stir/whisk every 30mins for about 1½ hours.
Pick through all the berries, hull the strawberries and red and white currants. Place the berries in a large bowl and douse with Cointreau. Leave covered with cling film to macerate for 1-1½ hours.
To serve, simply ladle out the berries into four chilled bowls. Scoop the sorbet and place into the centre of the bowls. You can serve a biscuit with this dish - for example, cantuccini or brandy snaps. Garnish with a sprig of mint and icing sugar. Serve immediately.