Thanks for that message. What happenned originally was, I made the jam boiled it for ages, and added some pectin, but it didn't set. So the next day my mother boiled it all up again, for some time, and then put it in the jars again, and this time it set real hard. It's a bit like soft fudge in consistency, a real knife bender and toast ripper!
I have tried warming it up and adding a bit of the runny marmalade that didnt get grabbed for the second go, and it has reset perfectly. Just right. Trouble is I have about 10 big jars of the tough stuff. But I will try what you suggested and let you know how it goes.!