Add the plums and measurement water to the preserving pan (there's no need to stone or slice the plums first). Bring to the boil, then cover and cook gently for 30 minutes, stirring and mashing the fruit from time to time with a fork, until soft.
Allow to cool slightly, then pour into a scalded jelly bag suspended over a large bowl and allow to drip for several hours.
Measure the clear liquid and pour back into the rinsed preserving pan. Weigh 500 g (1 lb) sugar for every 600 ml (1 pint) of liquid, then pour into the preserving pan. Add the peppercorns and heat gently, stirring from time to time, until the sugar has dissolved.
Bring to the boil, then boil rapidly until setting point is reached (10–20 minutes). Skim with a draining spoon or stir in butter if needed. Allow to stand for 5 minutes so that the peppercorns don't float to the surface.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, this jelly works well with roast lamb and roast potatoes.