Put the water, sugar, lemon juice, and vanilla pod into a wide saucepan and heat gently to dissolve the sugar. Bring the pan to the boil and let it bubble away for about 5 minutes, then turn the heat down to a fast simmer.
Cut the peaches in half, and if the stones come out easily then remove them, if not then you can get them out later. Poach the peach halves in the sugar syrup for about 2 to 3 minutes on each side depending on the ripeness of the fruit. Test the cut side with the sharp point of a knife to see if they are soft, and then remove them to a plate with a slotted spoon.
When all the peaches are poached, peel off their skins and let them cool (then you can remove any remaining stones). If you are making them a day in advance then let the poaching syrup cool and then pour into a dish with the peaches. Otherwise just bag up the syrup and freeze it for the next time you poach peaches.
To make the raspberry sauce, liquidize the raspberries, confectioners' sugar, and lemon juice in a blender or a food processor. Sieve to remove the pits and pour the puree into a jug.
To assemble the Peach Melba, allow 2 peach halves per person and sit them on each plate alongside a scoop or 2 of ice cream. Spoon the raspberry sauce over each.