I love grapefruit marmalade. There is something about the tangy flavour which is so much nicer than orange marmalade, and one of the recipes I found was this one, which makes amazing grapefruit marmalade! Sorry that it's in Imperial measurements as it's an older recipe - you will have to convert it to Metric if you don't do Imperial.
2 medium sized grapefruit
2¼ pints water
2¼ lb sugar
juice of 1 lemon
Cut the grapefruit finely, removing all the pips as you do.
Put the pips into a piece of muslin or a (non-paper) teabag/coffee filter type thing.
Soak peel and pulp overnight in the water with the pip bag in a large plastic or ceramic bowl, cover with clean tea-towel to keep it clean.
Next day transfer all the fruit, water and the pips into a large covered pan and simmer slowly until peel is quite soft. This should take about 1½ hours, but may be a little longer if you have cut the peel thicker.
Take the pip bag out of the pan.
Stir into the pan the sugar and the lemon juice. It helps the sugar dissolve if it's a little warm rather than really cold, room temperature is fine.
Bring the mixture to the boil, and boil rapidly in an uncovered pan until setting point is reached. This usually takes around 20 minutes. Start testing for setting with small quantities after 10-15 minutes. Setting temp is c. 220°F. If you don't have a sugar thermometer then a cold saucer/plate does the job just as well. Pop it in the fridge to make it really cold, then spread a teaspoon of marmalade into a level splodge onto it and leave it for a minute or two, then push your finger across the surface of the marmalade. If it's at the right temp then your finger will leave a trail across the marmalade. If it's not ready keep boiling and re-test at 5 minute intervals till setting point is reached.
Transfer into hot sterile jars, add a wax paper disc and seal the jar. Label and date. Leave till completely cold, then enjoy! Store opened marmalade in the fridge to prevent mould growth.
Takes 1½ - 2 hours