Place the mulberries and blackberries into a large, heavy saucepan and crush them with a potato masher.
Add the sugar and pectin and heat very slowly, stirring all the time until every grain of sugar has dissolved. Add the butter, increase the heat, bring to a full rolling boil and boil for exactly 4 minutes. Remove from the heat, cool for 1 minute, then place in jam jars and cover immediately.
Recipe by The Knobbly Plate at http://theknobblyplate.co.uk/?p=115