Generally, it's advised to add one tablespoon of lemon juice per pound of fruit. However, if you follow the jam recipe closely, you may wish to use bottled lemon juice. Bottled lemon juice has a ...
I recommend using damson plums if you can get them. They make the best jam - rich, deep in colour and with a bit of lemon for a little tartness. Scrummy on toast, scones, brioche or a bakewell tart.