Cut the fruit into small pieces and cook in the water until quite tender. Rub through a fine sieve, then weigh the pulp and add an equal amount of preserving sugar. Return to the pan with the strained juice and grated rind of the lemon. Cook over a low heat, stirring continuously until very thick. Allow to cool slightly, then pour into shallow trays lined with greaseproof paper. Leave in a warm place, such as an airing cupboard or plate-warming oven of a kitchen range, for three days, until it is quite dry. Remove the greaseproof paper and cut into crescent shapes. Roll in caster sugar. Store in a tin in layers, separated by kitchen paper.