Sprinkle the agar agar over the water and leave to soak for around 10 minutes.
Squeeze the oranges for the juice and thinly pare the rind of one of them.
Strain the juice and mix it with the sugar, rind and 340 ml of water in a large saucepan Now place the bowl of water which contains the gelatine/ agar agar in a double boiler and stir until the gelatine completely dissolves.
Bring the contents of the saucepan to a slow boil and stir till the sugar dissolves Reduce the heat and incorporate the gelatine solution too into the boiling water and simmer for 10 minutes or so.
Strain the liquid jelly into a wetted mould and stir in the rum (if you choose to).
Refrigerate to set for 1 1/2 hours.
Dip the outer side of the mould in hot water and turn it onto a plate and shake till the jelly slips out.
(YOu can cover the mould with the plate and then invert and shake to loosen) Decorate with segments of orange peel.