Whip 150ml (¼ pint) double cream and 150g (5oz) yogurt in a bowl.
Finely grate the zest from 1 orange and stir into the cream mixture, cover and keep in the fridge.
Squeeze the juice from the zested orange, and stir in 2 tbsp Cointreau.
Remove the peel and pith from 4 oranges. Slice them thinly, put on a plate. Pour over the juice mixture and leave in the fridge for an hour or so.
Just before serving, grind together a small handful of mint leaves with 2 tbsp sugar in a pestle and mortar (reserving a few whole mint leaves).
Scatter the mint sugar over the oranges with the leaves, and serve with the cream.