Mix the sponge ingredients together in a bowl, making sure they are well mixed. cover the bowl with a cloth or cling wrap and leave in a warm place for around 24 hours. You should see it noticeably grown in size and you may see bubbles form.
After the rise , mix in the additional flours, salt, molasses and water. Tip the mixture out onto a floured surface and knead for a minute or two. It should be soft but not sticky.
Dust a bread basket or line a bowl with a clean cloth and dust with flour. Form the bread into a ball and put in the basket or bowl. Cover and leave to rise in a warm place for 2 hours.
When ready to bake, dust a baking sheet with cornflour , put the loaf on top and score a couple of lines or a design with a razor or sharp knife. Let it sit as you preheat the oven to 425F/ 230C ( or little higher if your oven will go higher). Put a small baking dish in the bottom of the oven and boil some water.
When the oven has heated, put the loaf in on the baking sheet in the middle. Add some boiling water to the baking dish to create steam then close and bake for around 35 minutes, possibly slightly longer, until it is a nice even brown and is hollow sounding when you tap .
Transfer the loaf to a cooling rack and allow it to cool completely before slicing.