Put the chocolate pieces into a heatproof bowl set over a pan of simmering water. Add the rind, syrup and butter and leave to melt slowly, without stirring. Don’t let the base of the bowl touch the water. Tip the biscuits into a freezer bag or similar and bash into bits with a rolling pin – don’t make them too fine. Tip into a second bowl, then add the dried fruit and flaked almonds. Remove the melted chocolate from the heat. Stir until smooth then pour into the biscuit/fruit mix. Stir briefly until well coated.
Line a 20cm/8in shallow cake or brownie tin with clingfilm (lay two pieces across in a cross shape, carefully moulding them to fit the base, corners and sides, bringing them up over the rim). Tip in the cake mix and spread it gently until even. Leave to cool then put it in the fridge. It should be ready in about 2 hours. To serve, carefully invert the cake onto a plate or board, remove the film and cut into small squares.