Mandarin jam and ruby grapefruit jam

Ah, jam. Came into possession of a big pile of clean, washed jars the other day, and what better way to make use of them than through the wonder that is jam?
Making jam always makes me think of Isabella Beeton, in her glory days during her 20s as the original domestic goddess, rabbiting on about draining fresh apples from your local orchard to make cider and apple sauce and all the rest of it.
Anyway, so in the spirit of things, I decided to make use of the current glut of citrus round the place to do some jamming and preserving.
I must say at this point that although I have been making jam for Some Time, I am not very good at it. This is why I usually only make marmalade, as it is relatively idiot-proof and the natural pectins in citrus fruits help it to jam and set naturally. The jams make good gifts, and I sometimes flavour them with stuff like ginger, vanilla, passionfruit and chili... just to make things interesting.
For this occasion I decided to double-idiot-proof the recipe and to use CSR Marmalade Setting Sugar. It's got extra pectin in it, so your jam's got no excuse. This is a fairly smooth marmalade (cos it's easier that way...) but if you want to get all special, you can finely slice some of the fruit peel and have little slivers of peel in your jam, old school-style.

2 kg citrus fruit, whatever you like (peel grapefruits: leave oranges, mandarins, limes and tangelos whole)
1 kg CSR Marmalade Setting Sugar
A knob of butter
Water

Prepare your fruit. Throw it in the food processor (skins and all for anything that's not a grapefruit) and puree to almost smooth. Pour into a big heavy saucepan with 1.5 cups of water and bring to the boil. Boil moderately for about 15 minutes. This softens the fruit and starts releasing all the jammy goodness.
Bring the heat down to heat, pour in the sugar. Stir till all dissolved. Add butter knob. Bring it back to the boil and boil hard for 10 to 15 minutes or till it will set.
(You can do the old 'will-the-bloody-jam-set-yet' test by putting a plate in the freezer, and then when you want to test it, put a few drops of jam on the cold plate and leave it for a minute. If it's ready, the jam will form a skin that wrinkles when you push it with your finger.)
Note: if you don't use the special sugar and just use normal, it will still be fine... it will just take another half an hour or so to set.
The seeds can be a problem: I usually just scoop out any big ones I see that made it past the processor blades. They won't be awful to eat- they'll be soft and sweet just like the rest of the jam.
Decant the jam into sterilised jars (washed and rinsed in hot water) and leave for a few minutes before sealing.
Then line up all the jars in a row and admire them, before you start handing them out as presents!