Time for a little hardcore.

I had a lot of broken up bits if rubble to place in the bottom. A hammer was used to smash up and compact it down. I was only going to cast a shallow concrete base so it was very important to ensure the ground was firm.
After the rubble I spread some crushed lime stone into the cracks and compacted further.
Shuttering was placed across the open doorway and held with wooden pegs. We were now ready to cast the base. I left it for a week to settle some more.