Ripe plum

Soak in warm water for 20 minutes, then use 50–60% of the weight; more concentrated and darker, so reduce sugar slightly.

Use an equal weight of ripe pear, peeled and chopped; add a squeeze of lemon as pear is less tart and juicier.

Only if sourness is the point: use about 1 tsp per 100 g plum and taste; it will not give plum sweetness or body.