In a medium saucepan, stir together the honey and treacle. Bring the mixture to a boil, remove from heat and stir in the brown sugar, egg, lemon juice and lemon zest. In a large bowl, stir together the flour, bicarbonate of soda, cinnamon, cloves, allspice and nutmeg. Add the treacle mixture to the dry ingredients and mix well. Stir in the citron and hazelnuts. Cover dough and chill overnight.
Preheat oven to 180 degrees C. Grease baking trays. Using a small amount of dough at a time, roll out on a lightly floured surface to .5cm thickness. Cut into small rectangles and place them 2cm apart on the prepared baking tray.
Bake for 10 to 12 minutes in the preheated oven, until no imprint remains when touched lightly. Brush the icing over the biscuits while they are still hot and quickly remove them to wire cooling racks. Store in airtight container for a few days to mellow.
To make the icing: Combine the sugar and water in a small saucepan. Heat to between 110 to 115 C, or until a small amount dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface. Remove from heat and stir in the icing sugar. If icing becomes grainy while brushing biscuits, re-heat slightly, adding a little water until crystals dissolve.