Cut the grapefruit in half, loosen the segments and turn out into a basin, squeezing out all the juice from the fruit as well. Slice the oranges in half and prepare in the same manner as the grapefruit, adding the juice of the oranges to the basin.
Strain the fruit juice into a small saucepan, add the castor sugar and honey and stir over low heat until both sugar and honey have dissolved. Pour this syrup back over the fruit and chill thoroughly for several hours to emphasize the flavours.
Serve the fruit mixture spooned into four individual serving glasses – for a decorative effect frost the rims first. Run the cut edge of a lemon round the rim of each glass then dip the rim downwards into a saucer of castor sugar. As the rims dry they will turn frosty and white. Do not fill with the fruit mixture until ready to serve as contact with the moist fruit will make the sugary rims damp.