Beat the mascarpone until smooth, then gently stir in the custard and transfer the mixture to a serving dish. Scatter the berries on top.
Place the sugar and measurement water in a small, heavy-based saucepan and slowly bring to the boil, carefully swirling the pan from time to time. Keep cooking until the sugar dissolves, then turns a deep caramel colour. Pour over the berries and leave for a few minutes to harden.