Measure the water into a small saucepan, then cut the thinly pared orange and lemon rind into tiny shreds. Add them to the water along with the sugar, then gently bring the mixture to a simmering point and simmer gently for 20 minutes. Meanwhile, squeeze the orange and lemon juice into a bowl, and measure out the sherry, rum and brandy to join it. As soon as the 20 minutes are up, whisk the butter and flour paste into the contents of the saucepan. Bring back to simmering point, still whisking until the sauce has thickened. Now, keeping the heat very low, add the alcohol and fruit juices and, stirring gently, allow everything to become very hot without coming to the boil.