Pit the cherries and add them to a small saucepan with the caster sugar, heat on low until the sugar has dissolved, the cherries have broken down and the juices have been released (about 10 minutes).
Set a sieve over a small bowl and pour over the reduced cherries. Use the back of a spoon to push the cherries through the sieve.
Using a handheld whisk or stand mixer whisk the double cream until very thick. (3-4 minutes) You can do this by hand but it will take longer. When the cream is very thick, slowly add the condensed milk and whisk until fully incorporated, creamy and smooth.
Add half the reduced cherries to the creamy ice cream mixture and mix well - the mixture will turn pink.
Swirl through half of the remaining reduced cherries.
Repeat by adding the remaining ice cream mixture and reduced cherries, transfer to a 2lb loaf tin (or similar).
Cover tightly with cling film and freeze until the ice cream is your desired consistency.
Using these quantities will fill a 2lb loaf tin which translates to approx 1.7 litres of ice cream.