Just an update for anyone else with unripe plums.
I made jam and chutney.
The Victoria plums were hard and green but i resisted adding more sugar than the plum jam recipe stated and its fine, not tart at all and it set easily.
I tasted the chutney towards the end of cooking and that was very tart, perhaps because the cooking apples included were also under ripe, so i added a bit more sugar. Its really delicious. I wouldn't normally use plums for chutney as i have plenty of other things to make chutney from so this is a nice addition.
I have another load of plums coming next weekend and they will be 14 days riper so i can make the crumbles and tarts with those :happy: