Mix the condensed milk, lemon extract and lemon zest together in a mixing bowl
In another bowl, use an electric hand mixer to whip up the double cream to soft peaks
Gently fold the cream into the condensed milk mixture using a spoon or spatula
Layer about a third of the ice cream mixture into a 2.5 litre tub, and add blobs of lemon curd all over, then use a cocktail stock to swirl them around
Add more ice cream mixture on top, and more lemon curd, and keep going until everything is used up
Put in the freezer overnight or for at least 4 hours to set