Put all the mulberries, strawberries, raspberries or loganberries in a preserving pan or a large, heavy-based 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 pot and cover immediately.