Rum, quince and treacle? What was she, a saint???
My mum never made fruit cake (sniff sniff), but a good tip is to soak the fruit well for about 12 hours beforehand, that way you get a lovely moist cake, and one that doesn't crumble to buggery when you try to eat it.
Another way to moisten it is this, when cooked, prick the top with a skewer and store in an airtight tin for a week or so. Every day, pour a measure of alcohol of your choice over it. It should sink into the holes. Turn the whole tin over every other day, this should make the alcohol soak through all the cake. When the alcohol starts to puddle in the bottom of the tin, or it sets off the smoke alarm, that is the time to stop adding the alcohol. Be warned: I did this to my Christmas pudding last year, and it took 20 mins for the flames to die down when we lit it in the traditional manner. But it did taste wonderful!!!