So, we planted one side of our new garden with grass already. On the other side (the eastern, and thus west-facing, side), the first quick win was to move the pots of tomatoes and herbs from the old balcony. The west-facing fence was ideal for tying up the tomatoes, and it immediately made the space look more garden-like.
My parents also brought along four or five large polystyrene tubs (from frozen food deliveries) and some bags of compost, so I used those to plant a few turnips and some rocket and mustard greens, all of which do well in late summer. Another good quick win.
My intention is to construct a bunch of raised beds, as the ground underneath the paving slabs is compacted London clay, which hasn't been cultivated at all for at least a century or so. Instead of trying to improve it enough to grow directly in it, it's easier to put compost on top and let the soil underneath improve gradually. (It's also safer, as we don't know what the ground might be contaminated with, other than certainly lead, like any other patch of inner-city ground.) However, I want to think about the placement of these for a bit, and observe the flows (of sun and frost) in the space, before I do anything too much.
I decided that a single raised bed would be OK: at worst, it would be manageable enough to dump all the soil out of it next spring and relocate it. So I turned a deconstructed pallet into one raised bed, and took out the paving slabs and weed matting under that area. That left me with about an inch of sand, and compacted London clay underneath. I stuck a fork into it all a few times, then dumped some half-rotted leaves and about 100l of old compost/new compost/'soil improver' (cheap from the Southwark waste recycling centre) on the top of it.
In deciding what to plant, I took the polyculture annual veg approach I learnt in a session on the EAT course. There's a limit to what will do well planted in late summer, but I started off with a few kale and broccoli raab plants, which by now have a few small edible leaves. A couple of weeks later, I planted a few transplanted turnips (success: middling), some chard, and a lot of rocket and other mixed greens. The salad seedlings are coming up well now, and in a week or so I'll be able to harvest the first baby chard leaves.
In late September, I planted a row of spring cabbage towards the back of the bed -- it may be too late for these to do well, but we'll see; I might get lucky with the weather. Later this month I'll plant some garlic around the edges (good for keeping the pests off), and in November, some broad beans wherever there's a bit of space.
It was definitely worth the effort – even if I end up having to move it in the spring – to have that bed already in place and seedlings growing for the winter. It feels something like a proof-of-concept, or perhaps just a promise to myself that there will be a lot more of these by this time next year.
 The house was built in the mid-1990s, on land that used to be occupied by a warehouse, which was there from at least the early 20th century; we're not sure exactly when it was knocked down, but at best the land will have just been derelict for a bit before the estate was built. Prior to the warehouse, there was either housing, or possibly boat-building, going on in the area.