Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Lightly grease a 38cm x 26cm Swiss roll tin and line the base and sides with baking paper.
Stir the cake mix, oil, water and eggs together gently in a bowl. Add the espresso powder, then whisk (by hand or with an electric mixer) for 2-3 minutes, until smooth and creamy.
Pour the cake mixture into the tin and use a palette knife to spread the mixture to the corners of the tin and to level the surface. Bake for 20-23 minutes until risen and springy to the touch. Leave to cool in the tin; about 30 minutes.
Turn the cake out onto a large cake board and cut widthways (across the shorter edge) into 6 equal strips. Freeze for 1 hour.
Spread a quarter of a tub of icing onto 3 of the strips, then top each with 2 tablespoons of cherry conserve. Gently press the remaining 3 cake strips on top. Spread a thin layer of coffee icing over the top and sides of each sandwiched strip (leaving the ends un-iced) to seal in the crumbs. Chill in the fridge for 30 minutes.
Spread the rest of the coffee icing over the top and sides of each strip and chill for a further 30 minutes. Cut each strip into 6 slices. Spoon the melted chocolate into a paper or disposable piping bag and snip off the end. Pipe a chocolate zig-zag on the top of each slice. Leave in a cool place until set; about 20 minutes. Serve immediately. Store any leftover slices in an airtight container for up to 3 days.