Preheat the oven to 170°C (150 fan) | 325°F | gas 3 and place two 1 lb jam jars in the oven with their lids to sterilise them. Place a couple of saucers in the freezer; they will be needed for testing the jam's setting point.
Combine the raspberries and water in a large saucepan and cook over a medium heat, stirring occasionally, until the fruit softens. Once soft, add the sugar and stir well, scraping down the sides of the saucepan.
Reduce the heat and simmer the fruit and sugar for 10-15 minutes until the sugar has dissolved, stirring the jam occasionally. Once the sugar has dissolved, increase the heat and boil the jam for 10 minutes.
Remove the saucepan from the heat and spoon a teaspoon of the hot jam onto one of the cold saucers from the freezer; when the jam has cooled, prod it to see if a skin has formed. If it has, the jam is ready. If not, continue boiling for a few more minutes before testing again.
Once the jam is ready to set, add the butter and stir well until dissolved into the jam; skim off any scum on top and discard.
Remove the jars from the oven and place on a flat, heatproof surface. Spoon the jam into the jars and cover the tops with a waxed disc. Screw the lids on tightly and let the jars cool to room temperature before storing or serving.