500graspberries(I used frozen and thawed, as they are so much cheaper in this volume!)
1kgjam sugar(with pectin)
Remove the stones from the cherries, (either with one of those nifty cherry stoner gadgets or just a knife). Place the cherries and raspberries into a large bowl. Place a saucer or small plate in the freezer, (sounds odd, but you will need it later!).
I like my jams smooth and seedless, so at this point I blitzed the fruit to a juicy pulp with a hand blender then passed the fruit puree through a sieve into a large saucepan, discarding the raspberry seeds and little bits of cherry skin. (If you don't mind the seeds and skin in your jam feel free to skip this step!).
Add the jam sugar and lemon juice to the pan and heat very gently for 5-10 minutes until the sugar has completely dissolved. Then bring to the boil. If you have a thermometer, use it measure when the jam gets to 102°C. At this point place a small dollop onto your cold saucer/plate and leave it to cool slightly. Give it a nudge with your finger - if the surface wrinkles it is set, so remove the jam from the heat immediately. If you don't have a thermometer, start testing after the jam has been boiling for 5-6 minutes and repeat until it wrinkles.
Leave to cool in the saucepan for 10 minutes or so, then pour into sterilised jam jars, leaving the lids off until the jam is completely cool.