1. Put 250 ml of the water, the sherry, if used, lemon juice, sugar, lemon rind, and egg yolks into a pan, and whisk over a low heat until almost at boiling point. (Do not allow to boil or the eggs will curdle.)
2. Remove from the heat.
3. Soften the gelatine in the remaining water in a small heatproof container.
4. Stand the container in a pan of hot water and stir until dissolved, then whisk it into the saucepan.
5. Strain the jelly through muslin and pour into a wetted mould.
6. Chill for about 1 hour until set.