Start the blackberries over a low heat until all the sugar is completely dissolved, then bring to the boil and simmer for 5 mins.
Turn off the heat and spoon a little hot syrupy jam onto the chilled saucer.
Once it’s cool, push it with your finger. If it wrinkles a little, it’s ready and has reached setting point.
If it’s too runny to wrinkle, return the pan to the heat and boil in 2 or 3-minute stages, removing the pan from the heat each time you do the saucer check, until the jam wrinkles.