What you have is essentially a bank. The brick wall is retaining the soil above. I would go for a low simple planting with bushes closer to the back fence. Is it overlooked and do you own the land either side of the fences left and right? I wouldn't grass it as it looks difficult to access for cutting thereof. Possibly some sort of ground cover? You could do a mini terrace by dividing it up but it would be expensive to accomplish. Do you currenly have runoff down to the lower section when it rains?

So how do you access the upper area? You could plant come Vinca minor there as they quickly spread and will cover the ground. They will need watering until they are established. I would add planting in front of the wall such as climbers. Climbing Hydrangea is good as it self clings so won’t need trellis. Depending whether the area is sunny or shady, you could plant some shrubs like Photinia red Robin (evergreen and tall), Viburnum (also evergreen and tall). For colour you could add roses,but they like some sun. You could then add shorter plants at the front of the shrubs like Geranium Rozanne. Geraniums are pretty tough and come in so many colours such as pink, blue or purple. Below are Climbing Hydrangea, Vinca minor and Geranium Rozanne.

I might tier the area, perhaps even starting infront of the wall, a bed 45cm high with a ledge for additional seating and some steps to the right corner.

Then above the wall i would create two/three more tiers, starting at the fence with sleepers i would plant a slow growing hedge like choysia, then level out some pathways in between the remainder, covered in membrane and gravel to make it easier to weed. if you need to hold back the levels then low fencing sold in metre strips should be sufficent

i always like planting acer dissectum garnet, creating beautiful mound shapes that suit a slope, perhaps three. then geranium ann folkhard with magenta flowers is a good combination and foxgloves and spring bulbs and autumn anenomes etc

What direction does this face and what’s the view like from the top of the wall towards the house? I can see a gin and Tonic deck above the wall!