Put the wine to chill several hours in advance. Rub the lump sugar over the rind of the well-washed lemon until the sugar is yellow and full of zest. Put in a jug and crush the lumps. Add the strained lemon juice and stir to dissolve the sugar.
Add the sherry and chilled wine, and stir to blend. Decorate with slices of cucumber. Makes 12–14 glasses