Place half of the raspberries in a blender and puree. Pass the liquid through a sieve to remove any lumps and seeds.
Heat half the cream with the sugar, vanilla pod and lemon zest in a heavy saucepan over a medium heat, until small bubbles appear around the edge of the pan. Remove from the heat and set aside to infuse for 20 minutes.
Meanwhile, oil 6 ramekin dishes.
Place the milk in a small saucepan and sprinkle over the gelatine. Remove the lemon zest from the cream and discard. Remove the vanilla pod and scrape out the seeds. Add the seeds to the cream.
Heat the gelatine over a low heat until it is liquid (do not boil), then stir into the warm cream. Place the saucepan in a bowl of iced water to help it cool. Stir occasionally.
When the vanilla cream has reached setting point, whip the remaining cream until it just holds its shape. Fold together the whipped cream and vanilla cream, adding whichever is thinner to the thicker mixture. Next add the strained raspberry liquid.
Pour into the prepared moulds. Tap them on a work surface to release any air bubbles and refrigerate for about 4 hours until set.