Wash and dry the lemon balm and put into glasses.
Slice off four slices of lime and put into the glasses.
Squeeze the rest of that lime and the other lime.
Put the lime juice into a pan with the grape juice and sugar and bring to the boil.
Boil for 4 minutes, stirring, then test for setting point.
Add the Grappa and pour into the glasses.
Leave to cool for at least 2 hours.
Alternatively pour into jam jars, seal well, stand the jars on their heads and leave to cool.