Here's the recipe:
8 oz caster sugar
Put the rasps and the sugar into a food processor and purée until smooth.
Push through a plastic sieve into a bowl to remove all the seeds.
Put into the freezer - in the bowl - and freeze for about 30 minutes.
Take out and beat with a hand-held electric beater and replace in the freezer.
Repeat the 30 minute freezing/beating until you get the desired consistency. (I find that after about 3 times, it's good to go.)
Put into a plastic box and freeze.
Take out of the freezer for about 5 minutes before serving to allow it to soften up.
I usually make double quantity but, as my food processor won't take more than the 1lb/8oz quantities at a time, I just do it in two lots and combine them at the sieving stage.
Be warned - this is very moreish. You have no idea how hard it is to sit here typing this knowing that there's raspberry sorbet in the freezer! :oD