Make a syrup with the water and castor sugar by dissolving the sugar and bringing to the boil. Simmer fast for 5 minutes.
Bring the quinces, granulated sugar and water to the boil and simmer until quinces are cooked. Pass the pulp of the quinces through a fine sieve. To the pulp, add the lemon juice and the finely shredded mint.
Combine the sorbet syrup and the quince pulp and p