Beat the cream in a deep, narrow bowl until quite stiff - it should not be at all runny. Pour some sherry into a shallow bowl. Have the plate on which you are going to serve the chocolate log ready - preferably use a long oval or rectangular serving plate which is at least an inch or so longer than the length of the packet of biscuits.

Take one of the biscuits and dip it into the sherry to completely wet both sides (but don't leave it to soak in the sherry), then spread a thick layer of cream (about the same thickness as the biscuit) across one side of the biscuit. Dip the next biscuit into the sherry and push gently onto the cream on the first biscuit. Once you have 2 or 3 biscuits 'sandwiched' together with cream, they should be able to stand up on edge by themselves on the serving plate - start working from one end of the plate. Continue to add alternate layers of cream and sherry-dipped biscuits until the plate is full or the biscuits are all used. Cover the log completely with some or all of the remaining cream, then place in the fridge for about 12 hours. Decorate with chocolate shavings when ready to serve.