There are many, many, many recipes for sourdough starters. Had I saw Paul Hollywood's recipe first I would probably have used that, but I didn't; so I used this one instead.
So, the main protagonists in day 1 are: 5 tablespoons of live full fat plain yoghurt and 6fl oz skimmed milk. Gently warm the milk and mix into the yoghurt, pop into a airtight container, I used a kilner jar, place in a warm place for 12 - 24 hours to thicken. Here is how it looks now.