I use my mother's recipe, which she had from a friend of my grandmother's. One year when we were moving, I realised that stuff I had 'boxed up early' included her recipe book.
So I went to the blessed Delia, to discover the same recipe - I compared them side by side the following year, and the only difference is the use of muscovado or demerara sugar!
In recent years I have varied the fruit, by including things like dried cranberries to replace some of the raisins.
One year, when I was about 10 or 11, my sister and I raised funds for our church by baking a Christmas cake, and challenging 'guess the number of currants'. We put just 1 in, which was when I realised you could play around a bit with fruit quantities. I live in such a nice old-fashioned place, that the story is still told occasionally!