Remove the zest from the grapefruit, cutting off and discarding any white pith. Slice into julienne strips. Blanch in a small saucepan of boiling water for 10 seconds and drain.
Heat the water and caster sugar in a small heavy-based saucepan over low heat until the sugar syrup reaches simmering point. Add the grapefruit zest and cook over low heat for 20 minutes o