Place the raspberries in a large saucepan and heat gently until the juices begin to run and the fruit softens.
Add sugar and heat gently, stirring continuously until the sugar dissolves. Do not allow to boil.
Add the knob of butter.
Still stirring, increase the heat and bring to a full rolling boil, one that bubbles vigorously, rises in the pan and cannot be stirred down.
As soon as this stage is reached, start timing.
Boil for 4 minutes only. Remove from the heat. Pot and cover in the usual way.
Microwave Instructions: Place the fruit in a 2.8 litre (5pint) bowl and microwave on high uncovered for 4 minutes. Stir in the sugar and butter.
Microwave on HIGH uncovered for 14-16 minutes, stirring halfway through. The mixture must boil vigorously for 4 minutes during this time. Pot and cover in the usual way. (Based on a modern 650W microwave oven, decrease cooking time for higher wattage ovens.)