1 In a pan, dissolve sugar in 300ml (½ pint) water set over a low heat. Increase heat and boil for 1 minute. Pour into a large jug, cool, then chill.
2 Stir in gin, citrus zest and juices and tonic water then pour into a freezer-proof plastic container and freeze for 5-6 hours until solid.
3 Break ice up then pile icy chunks into a food processor. Froth egg white with a fork, add to processor and pulse quickly to mix. Pour back into container and freeze until solid.
4 Serve 3-4 small scoops per person and decorate with zest and citrus slices, if you like.