Clean and prepare all fruit except the prunes. Pick over vine fruit and remove any stems. Chop the larger fruit into small pieces (a pair of scissors does the job more easily than a knife). Mix the fruits in a bowl and add brandy and sherry. Cover tightly and leave overnight or longer.
Prepare one deep 23 cm square cake tin (or, for one cake to keep and one to give, one 20 cm square tin and one 15 cm square). Grease the interior of the tin(s) with butter. Line with two layers of brown paper cut to fit and to project above the sides of the tin by about 5 cm.
Inside the brown paper, fit a layer of grease proof paper, projecting similarly. butter the inside of the grease proof. Pit and chop the prunes. Cream the butter and sugar well. Add the eggs, one at a time, mixing each in thoroughly. Add prunes.
Sift flours together and add the butter-sugar mixture alternately with the fruit. Add the essence and coffee.
Put the mixture into the prepared tins(s). Dip one hand in cold water and pat the surface of the cake flat. (The thin layer of water keeps the surface moist for a long time and lessens the chance of the cake rising in the middle.) Bake in a preheated 150C oven for 1 hour, then reduce the heat to 135C and cook for a further 2 1/4 hours.
Take the cake from the oven and fold the extra paper layers over the top of the cake, tin and all, while still hot in at least 4 layers of newspaper. Place on a rack and leave overnight. (This process keeps the cake moist and prevents it cracking on top while it cools.) In the morning, remove from the tin, peel off the paper and store the cakes(s) in an airtight tin.