Hull the strawberries, wash and cut in half if large or leave whole if small. Put into a preserving pan with the lemon juice and sugar and leave for 1 hour, stirring occasionally. Place over a low heat and stir continuously. When the sugar has dissolved, add the butter which reduces foaming. Bring to the boil and boil rapidly for 4 minutes, stirring occasionally. Remove from heat, add the pectin and stir well. Allow to cool for at least 20 minutes before bottling to prevent fruit rising to the top. Pour into hot sterilized jars and cover.