Add the grapes and their stalks to a large preserving pan. Squeeze and reserve the juice from the lemons. Chop the lemon shells and add to the pan with the measurement water. Bring to the boil, reduce the heat and cover the pan. Simmer for 1 hour.
Allow to cool, then pour into a scolded jelly bag suspended over a large bowl and allow to drip for several hours or overnight.
Measure the clear liquid and pour back into the rinsed preserving pan. Weigh 375 g (12 oz) sugar for every 600 ml (1 pint) of liquid, then pour into the preserving pan. Pour in the port and reserved lemon juice and cook over a low heat, stirring continuously, until the sugar has dissolved. Bring to the boil, then boil rapidly until setting point is reached (10–15 minutes). Skim with a draining spoon or stir in butter, if needed.
Ladle into warm, dry jars. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve drizzle the jelly over vanilla ice cream.