Hi Alisdair,
I think I forgot to reply to your posting re the size of the buckets I am using.
They are as you thought, the thick black plastic ones with handles. I am using various sizes from 75l down through 50l and then 40l. So far so good. No splits with neither the summer nor winter extremes of temperature. And the handles help with moving them when necessary. Holes are easily drilled into the bottom for drainage. I put a layer of horticultural fleece across the bottom to stop soil being washed out.
I have put them in tiers from the back to the front and on the front line I have put some trailing plants to cover the front of the bins e.g.
Helianthemum "Ben Fhada" (just getting big enough now to cover the front but worth the wait, I hope, If it flowers next year) and
Emilia coccinea (truly an annual? Still going strong for me though flowering marginally less now than in the summer and autumn). Some clay pots at the very front but I have bought some flat plastic builder's trays 20cm deep where I intend to plant a mix of high growing and trailing sedums to form the front line next year. It may be too hot but I have a lot of cuttings on the go. The parent plants will be kept more in the shade, as they were this year, so I shouldn't lose any type of plant altogether.
At the back there are tall bushes e.g.
Buddleja madagascariensis or tall growing climbers e.g.
Plumbago capensis I also train annual climbers up through the more robust perennials e.g. Ipomea quamoclit (summer flowering) growing up the stems of a yellow rambling rose (spring flowering) and
Clematis armandii (early spring flowering).
Maurandya barclayana is the fourth in the combination and still flowering even after the freezing temperature of one night last week.- thanks for the seeds Chantal
Here are some pics to give you an idea of how things are coming on but just to say that bins seem to work, are a quarter of the price of pots and, if covered in plants, are not too bad on the eye. There is one of the "garden" before I started and a couple of views taken this summer. The "Ben Fhada" can be seen a couple of weeks after it was planted last spring and as it is now.