Put the flour in a large bowl, add the yeast and salt. Stir in 400ml water and the oil, you should end up with thick, gloopy dough. If it is too stiff add a little more water. Scrape down the sides of the bowl, cover with a tea towel. Chill overnight or until you want to cook it, about 18 hours in all is perfect. If it looks like it might climb out of the bowl squash it down.
When you are ready to cook the focaccia bring the dough back to room temperature and heat the oven to 200C/fan 180C/gas 6. Brush a small roasting tin with olive oil. With oiled hands, fold the sides of the dough in one at a time in the bowl, tip the dough over onto the seam side in your tin and press it out. Leave to rise for 10 minutes. Poke dips into the dough, liberally drizzle with oil and sprinkle with salt. Bake for 40 minutes or till risen and golden.