Blood Orange Sorbet

I'm a little bit obsessed with blood oranges at the moment, and served this with a chocolate tart.  It's based on a recipe by Galton Blackiston.

275ml water
175g caster sugar
425ml of blood orange juice
juice of 1 lemon

Make a simple sugar stock syrup with the water and the sugar.  Bring to the boil, and once the sugar has dissolved, simmer gently for another 5 minutes.  Allow to cool then add to the orange and lemon juices.  Put into the fridge to chill.  Put the mixture into an ice-creamer maker, or into a shallow container, turning every half an hour or so to disperse the ice crystals.  Allow to soften slightly (for around 15 minutes) before serving.