Place plums in saucepan and add water. Simmer until plums soften.
Strain plums to remove the skin and seeds, pushing the pulp through the sieve.
Weigh the plum pulp and add up to an equal quantity of sugar (a little less is ok), bring back to the boil and stir to dissolve.
Add lemon juice.
Keep on a brisk simmer for 30 minutes, stirring occasionally to prevent sticking. Do not stir constantly or it will cook very slowly. When it is thick enough that the spoon leaves a trail in the mix and/or you can smell a slight burnt smell, it is done.
To store long term- jar. Alternatively, pour into a shallow loaf tin lined with baking paper, or silicone mini muffin tins, and refrigerate. Suitable to freeze.