Butter a 20x30cm baking dish and set aside. Butter a 3-litre saucepan.
Place buttered saucepan over medium heat and combine sugar, evaporated milk and butter within. Heat to between 112 to 116 degrees C or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Remove from heat and stir in peanut butter and marshmallow spread. Beat vigorously until smooth. Pour quickly into prepared baking dish. Let cool completely before cutting into squares.
this seems to be far FAR too sweet, 800g of caster sugar is a lot and then 200g of marshmallow fluff? very sweet. Mine has yet to set, it's been in the fridge for 6 hours now. Also this makes far far more than a 20 x 30cm dish (unless you want 3 inch thick fudge) so I used 2 large cake tins, 1 of which is actually in the freezer and 2 hours on, STILL has not set. I'm hoping it will set by the morning.
I'll come back and post what happens. - 18 Feb 2012