1. Cook the raspberries and icing sugar gently, until the berries begin to collapse, then remove them from the heat and sieve into a bowl to remove the pips.
2. In a large bowl, whisk together the condensed milk, cream and vanilla until it starts to thicken. This takes about 5 minutes with an electric hand whisk.
3. Pour some of the raspberry puree into the ice cream mixture and stir only enough to swirl through.
4. Line a loaf tin with clingfilm, then pour in a layer of ice cream, next pour over some puree, then another layer of each. Until you have filled your tin. Keep a little of the puree to pour on top of the ice cream.
5. Pop the ice cream in the fridge to set, preferably overnight. Take out of the freezer about 15-20 minutes before you want to serve it, to let it soften.