I made this for the 1st time last week (when grapefuits were 21p in Aldi!) and its worth making- a very pretty colour and good acidity/sweetness balance.

Be warned - like with seville - you get a LOT of marmalade for a few grapefuits! 

 Recipe:

3lb of fruit - you want about 2.5lb grapefuit and the rest lemons (but the actual balance ndoes not matter too much - aim 3/4 grapefuit and 1/4 lemons)

6lb sugar

1) squeeze fruit - do this over a jelly bag (i use this in a seive) over a jug to catch all the juice and catch the pips/pith in the jelly bag). Tie the jelly bag to the pan handle, so it hangs into the water.

2) Place halved fruit in large pan with 7 pints water and simmer for about 45-60 mins unitl skins are soft and water reduced to about half

3) allow fruit to cool (I usually leave it over night) and then remove the skin and slice thinkly. I put them in a large sive over the pan so no juice is lost

4) Squeeze the jelly bag into the pan - try to get as much of the juice/jelly out as possible.

5) add sugar, heat unitl sugar dissolved and then boil to setting point.

6) its worth taking the pan off the heat for few minutes before putting into jars and stiring to try to distribute the peel  (this will never be quite even!) 

7) Bottle, seal and label.