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Peel 2 long strips from the piece of cucumber and put in a small tupperware for transporting. Grate the remaining cucumber into a sieve set over a bowl and press down on it with the back of a spoon to extract as much juice as possible. Add the elderflower cordial and gin to the cucumber juice and put in a bottle ready for travelling. At your picnic, wind the strips of cucumber around inside 2 glasses, pour in a quarter of the cucumber gin mixture, and top up with with a quarter of the bottle of tonic water to serve.