Wash the 1200 g (2½ lb) rhubarb and 600g (1½ lb) apples chop intoo even sized pieces. Leave the peel, core and pips on the apples. Put the fruit into a large pan with 1 litre (1¼ pints) water.
Bring the fruit and water to a boil, then turn down and simmer until the fruit is soft.
Put a colander or sieve over a large bowl and line it with your jelly bag. I stretch my nylon jelly bag over the edges of my colander to keep it in place. Ladle the fruit pulp and liquid into the jelly bag until it is full.
Thread a piece of string through the loops on the jelly bag and secure it to something that will allow it to hang freely above the bowl. I use the door knob on one of my wall mounted kitchen cabinets. Leave the juice to drip through for at least 4 hours or overnight. Do not squeeze the jelly bag.
Measure the juice from bowl in a large measuring jug. You will need 450 g (1 lb) of sugar for every 600 ml (1 pt) juice.
Pour the juice into a large clean pan and bring to a simmer.
Add the sugar and heat gently until the sugar has all dissolved and no crystals (gritty bits) are left.
Bring to a rapid (rolling) boil and boil for 15 minutes before testing for set (see notes). If not yet set, then boil again for 5 minutes before testing again.
If your jelly is set then skim off any froth on the top of the jelly with a large spoon. This is quite edible but will spoil the look of your clear jelly.
Once the jelly is clear it is ready to be ladled into jars.
Use a jam/jelly funnel, a jug or a ladle to pour the jelly into sterilsed jam jars and seal with a lid. Leave to cool, the jelly will firm up as it cools.
Serve Rhubarb and Apple Jelly with bread, toast, scones and croissants.