The next step is to apply a proper layer of clay to the outside of the building. This is important mostly to prevent rodents and other creatures from getting into the clay. It also gives a base to attach the cladding to (using noggins):

In summary:

This shows the process in more detail. First I had to cover the timber with woodfibre board, so that the clay would stick to it. I should have done this before I added the clay slip! Then I oovered it with a good layer of pre-mixed clay. The next day or so I would go over it and smooth it out and add the noggins for the cladding.

It was good that I did the outside of building first, which would be covered by the cladding, as it gave me a chance to learn how to plaster in a way that no one will ever see!

Thanks to everyone who helped: