To help with this issue I dug cow manure and gypsum through the ground which was left to break down for a few weeks before planting the trees.
Gypsum is a soft mineral known as a calcium sulphite. It improves the soil by breaking it up allowing water drainage and making it easier for plants to take root. Cow manure adds nutrients to the soil and continues to improve the drainage as it breaks down into the ground. I used a lot of composted cow manure, digging it through the ground a couple of weeks before planting so that it had time to start breaking down.
I also found a lot of rubbish in the ground where the mandarin tree needed to be planted, and had to get as much out as possible without digging up the whole site. Luckily there was nothing dangerous or hugely damaging, and I was able to reuse some of what I found.
To help soak up the excess water I planted Canna Lily along the slope which serves as a windbreak, as well as banana trees in the boggiest areas of the garden.