Add the strawberries and rhubarb to a preserving pan, pour over the measurement water to just cover the fruit, then bring to the boil. Cover and simmer 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 then 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 lemon juice, heat gently, stirring from to time, until the sugar has dissolved.
Bring to the boil, then boil rapidly until setting point is reached (20–30 minutes). Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars. 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 makes a lovely filling for a Swiss roll.