Line a 2-litre bowl with a double layer of cling film, leaving some excess overhanging the rim. Cut the Swiss roll into 1cm slices and use them to line the base and sides of the bowl. Drain the cherries, reserving the syrup. Drizzle the Swiss roll slices with the kirsch syrup, saving a few tablespoonfuls for the top. Set aside.
Put the sugar and water into a small heavy-based saucepan and stir over low heat to dissolve. Increase the heat to high and boil until the syrup registers 120 ° C on a sugar thermometer. Meanwhile, in a clean large bowl, whiskey egg whites to stiff peaks.
When the sugar syrup is ready, gradually pour on the egg whites in a steady stream, whisking as you do so. Continue to whisk until the meringue has doubled in volume and the sides of the bowl no longer feels hot.
In another bowl, whisk the cream to soft peaks. Fold in the ginger syrup and orange liqueur, followed by the meringue. Taste and sweeten with a little more ginger syrup if required. Stir through the cherries, chopped ginger, glacé fruit, marron glacé, if using, and chopped pistachios. Spoon into the Swiss roll-lined bowl and level the top.
Cover with the remaining Swiss roll slices, cutting them to fit as necessary. Drizzle with the rest of the kirsch syrup, then fold the excess cling film over the top to seal.
Place a flat plate on top of one another. Chill for an hour, then remove the weight, wrap and freeze the bombe.
To serve, unwrap the bombe and place on a flat plate or cake stand. With sprigs of of the decorate redcurrants dusted with the icing sugar the if you like, or the serve just as with the IT is IT. Let stand at room temperature for about 15 minutes before slicing.