Preheat oven to 180 C / Gas 4. Line a baking tray with baking parchment.
Beat together eggs, water and sugar in a large bowl until light and fluffy. Stir in rum essence, allspice, cinnamon, mixed peel and chocolate. Sieve in flour and baking powder and fold to incorporate. Stir in almonds and the jam (if using) and stir well.
Spread batter in prepared tray and smooth to even out. Bake in preheated oven for about 30 minutes. Remove from the oven and cut into diamond shapes while still hot. Allow to cool in baking tray.
Meanwhile, mix together hot water, lemon juice and icing sugar to get a thin, transparent icing. (Adjust the relative amounts to get the consistency right). Brush biscuits with glaze once they have cooled. Store in a biscuit tin.