I laid the clay out in the sun. It will later have to be rehydrated but first needs to be dried out so that it can better absord the water. As you can see by the images, it contains a lot of unwanted material that will need to be removed. After the clay had been sat out in the sun for a few hours, I put it into a large metal bowl and broke it up further into small lumps. It's still quite damp so it will have to sit in the shed for a few days before I can filter it.
Having left the clay to dry out for a few days, I broke it up with the back of an axe and picked out the worst of the roots and rocks, and added water. The mix will now sit for a day or two to allow the clay to absorb the water and hopefully separate from all the other material, before being poured through a screen to filter it. Though there's still quite a lot of organic stuff in there, the clay's already looking nice and soupy, like a thin slip.