I found it!
Oh my goodness, it doesn't get much easier than this:
(I make a double batch because it disappears quite quickly)
1 can of Condensed Milk (395gms)
125 grams butter
375 grams milk chocolate
200 grams dark chocolate
Melt the ingredients together. You can do this on the stove top, on a medium heat, or in the microwave ( 1 minute at a time, stirring in between, being careful not to overdo it).
My daughters always make this recipe, and usually do it on the stove top as it is easy for them to monitor the melting.
Once it is all amalgamated together, pour it into a baking paper lined container - I use a rectangular plastic container, so I can put the lid on it and keep it in the fridge, or pour it into a flat baking tray.
It is difficult to cut, but it is so easy to make, and makes a wonderful Christmas gift.
I once made a white chocolate version of this, but found it tasted too sweet. You can also make it with all milk chocolate.I prefer the version with the dark chocolate in it.
Our friends, the Gospers, who passed the recipe on to us also make it with macadamia nuts in it - that is really delicious.