2kg of grapefruit 

2kg of sugar

2L of water

Juice of one lemon

Wash and slice the grapefruit very thinly with a sharp knife. Forget those fancy gadgets and do it by hand as this makes the best result. Place in a non metallic container and soak overnight... I don't know why, but dad did this too.  

Next day, tip all into a tall pot and boil for 40 mins quite quickly, then lower the temp and add 2 kg of sugar until dissolved  and then bring back up to a rapid boil.

Boil for one hour at a fast boil, stirring at regular intervals so that it doesn't stick but start testing for readiness at 50 to 55 minutes.

For me this is the tricky part.

I took a saucer from the freezer so it was cold so I could drizzle jam off the wooden spoon onto the saucer to test it. If a skin formed then it was going to set.

I had jars with seals at the ready, washed and sterilised either in the oven or with boiling water. I found that even the little half size Agee jars are good with seals and screw tops or I could reuse jam jars with seal lids.