To clean the jars, preheat the oven to 140C/275F/Gas 1. Wash the jars well in warm soapy water then rinse thoroughly under running water. Leave the jars and lids to dry, upside down, in the oven. (Or you can clean the jars by putting them through the hot cycle of a dishwasher.)
Place a few saucers in the freezer to chill. These will be used to test if the cooked jam has reached setting point.
Place the raspberries and sugar into a wide-mouth preserving pan over a very low heat, stirring every so often until all the sugar has completely dissolved. It's important to have the heat low so the sugar dissolves rather than melts and sticks to the pan.
Bring the fruit mixture to a rapid, rolling boil (when the bubbles cannot be calmed down by stirring with a spoon). Cook for 3-5 minutes until the jam reaches setting point. As your jam approaches setting point, it will thicken and start to boil more slowly, with thicker, heavier bubbles.
To test if the jam has reached setting point, first remove the pan from the heat while you test the jam – this is very important. Spoon a little of the jam onto a cold saucer, leave to cool for a few minutes, then push your finger into the jam. If it wrinkles, it is ready. If not, return the pan to the heat and cook the jam for a minute or two more and test again.
When the jam has reached setting point, carefully ladle it into the sterilised jars (a sterilised jam funnel is very useful) and twist the lids on while the jam is still hot. The jam will thicken up as it cools and the seal on the jars should dip. If a jar doesn't seal, store it in the fridge and use the jam within a couple of weeks.
Sealed jars can be kept in the cupboard for months, although after six months the flavour begins to deteriorate.