The slab and the retaining wall were a much simpler process. Especially since we ended up getting a bricklayer to do it for us.
Originally we had planned to lay the slab with the help of a friend, and build a pine retaining wall ourselves. But after researching it we realised a pine wall would only last about 10 years and would then probably need replacing, but a brick wall could last forever. So it was more expensive up front, but it was done right, and wouldn't rot away in the future and need redoing. Also, we were so over that area at the time, and had both gone back to work so it would have taken us multiple weekends. It took the bricklayers 2 mornings over about a week.
They came one day, and laid the slab, let it set for a few days and then came back and built the wall.
We lay some agi pipe between the soil and the wall. We made sure it had a slight fall so the water would drain away where we wanted it to go, and then covered it with drainage rock.
It already looks so much better, and tidier. As this is at the front of our house (but below street level-check out the first photo) we think we will plant something to hide the tanks. It will probably be in pots so we can cover up the clay and drainage, and finish the area off.