Put 12 clean glass jars and lids on a baking paper lined oven tray and put in the oven. When you start cooking turn the oven on to 100 degrees C. This sterilises the jars and makes sure they won't crack when you put the very hot jam in them.
Peel, core and cut apples into small chunks, the smaller the quicker they cook but having chunks to bite in your jam is nice too.
Put the apples and the water into a large saucepan. It needs to be big because the jam bubbles up nearly three times the volume. Cook with lid on over a medium heat for about 20 minutes; until soft but not mushy.
Add the blackberries to the stewed apples for the last 5 minutes.
Lower the heat and add the sugar and stir - without boiling - until it is all dissolved. Add the cinnamon and nutmeg to your taste preference.
Bring the jam to the boil and boil for about 20 minutes; stirring often. Don't put the lid on. Wear long sleeves and use the lid as a shield as it can spit boiling sticky jam and it hurts. Stir across the base of the pan to check its not sticking or burning.
When the jam falls from a tilted wooden spoon in 'thick sheets'; and is no longer runny; test for setting point. Using a sugar thermometer the temperature for jam is 104 degrees C.
Remove the jars out of the oven and either transfer the jam into a jug or just ladle it straight into the jars up to the start of the neck. With a clean, new, damp cloth clean the edges of the neck; inside and out. Seal with lid.
Once all are sealed then clean the rest of the jam off the jars and leave to cool. This can store in a cool, dark place for 6-12 months. Refrigerate after opening.
If you are making these for a present, skim off the scum in step 6. This is tasty and goes well on a bit of toast when its cool. This step is only for visual beauty there is nothing tastier than a bit of jam scum.
Testing Jam Doneness
To test the jam without a sugar thermometer remove the jam from the heat and place a little of it onto a cold saucer from the freezer. A skin will form on the surface of the jam and will wrinkle when pushed with your finger. If it's not doing this then put the jam back on the heat to boil and try again in a few minutes until it does. The jam is ready when this occurs.