Tip the pineapple pieces in natural juice into food processor and blitz to a purée. Pour into a pan and add the caster sugar and 200ml water. Stir over a low heat until the sugar has dissolved.
Take off the heat and stir in the juice of 2 limes and the reduced-fat coconut milk. Transfer to a measuring jug and add enough cold water to make it up to 1 litre. Cool.
Pour into a freezer-proof container and freeze for 2 hours, or until just set. Tip the mixture into a bowl and whisk with an electric beater for a few minutes, until smooth. Pour back into the container and freeze overnight.
Break up with a fork and serve in glasses. Decorate with washed raspberries and mint leaves.