Bring half of the mandarin juice to the boil along with the sugar, cook for approx. 2 mins., pour into a stainless steel bowl and leave to cool. Stir in the rest of the mandarin juice and the lemon juice.
Cover the mixture and freeze for approx. 6 hrs., vigorously stirring 4 times.
Finely chop the chocolate, melt in a thin-sided bowl over a gently simmering bain-marie, stir until smooth. Roughly chop the nuts, mix together with the flour, sugar, vanilla and ground cloves, add to the chocolate along with the water, mix. Crumble the mixture, transfer to a baking tray lined with baking paper.
Bake for approx. 12 mins. in the centre of an oven preheated to 180°C. Remove from the oven, cool on a cooling rack, chop a little more finely.
Scoop balls of sorbet and plate up, sprinkle with the crumble.