1 hr 10 min.
Put a saucer in the freezer. Scrub the clementines under the hot tap and then blanch them in boiling water for 5 minutes before refreshing in a bowl of iced water.
Top and tail each one before coarsely chopping them in a food processor.
Pour the sugar, 200 ml water and the lemon juice into a heavy bottomed preserving pan and boil for 5 minutes. Add the fruit and leave to cook for 40 minutes simmering gently, stirring from time to time until you are happy with the consistency. To test, spoon a little jam onto the frozen saucer, then push the side of it with your finger - if the surface wrinkles; it's ready.
Pour into sterilised jars and screw the lids on tightly before leaving to cool.