We collected 4 tubs of the stuff a couple of weeks ago and stewed some in a crumble and turned some into jam.
1 ice cream tub is about 3lbs and you need that much to 1 kg pack of granulated sugar.
Do 1 tub per batch at a time. Stew the damsons until they "bleed" and the stones are loose from the flesh, mix in the sugar and bring to boil, simmer for about 20 minutes until the sugar has dissolved then rapidly boil for about 15 minutes or so. It's done when you put a blob on a cold plate and it wrinkles as you push it when set.