Just realised it's one kg of sugar to one kg of blackberries, not two, like the marmalade recipe.
Rats. Tastes a bit sweet, but we'll see if it's edible in the morning. Otherwise the dog and I will have to pick some more. Took me a while to realise that he was emptying the tub almost as fast as I was filling it. Once I'd explained to him how the system worked, he went off the pick his own, from those near ground level that I won't pick, as they are in dog leg-cocking range...…. He's not as fussy.