squeeze the lemon and set the juice aside

chop up the peel and add it to a pan with the grapes and the water

bring to the boil

reduce the heat, cover and simmer for 1 hour

strain through a chinois, gently pressing all the juice out of the grapes

add the juice, the lemon juice, the sugar and the port into a pan

cook over a low heat, stirring continuously until the sugar dissolves

bring to the boil and continue boiling until it reaches setting stage

this should take about 20 minutes

remove from the heat

skim off any scum carefully using a slotted spoon

pour into a warm sterilized glass jar

place a wax paper disc, wax side down on the jelly

seal with a lid and store in a dark, cool place for up to 3 months