Good heavens that recipe for sloe jam is awful. it will be full of stones.
Sloes are one third fruit and two thirds stone or pip,you need to make sloe jelly not jam.
No suggestion for how to reach a setting point and to be a jam you need equal quantities of a fruit like sloe and apple to sugar.
So cook your sloes and apples or crab apples with water to cover, drip through a square of muslin next day measure the juice and for each pint add 1 pound gran sugar, dissolve bring to rolling boil until a set is reached[test by putting a spoonful on a cold saucer and pushing with your finger,when it wrinkles it is ready]
Pour into small cold sterile jars, cover and leave for 24 hours before moving.