I'm happy to report that the little elder tree I dug up and potted back in June is doing well.
At first I didn't think it was going to survive- the top third of it flopped over, it lost half of its leaves and the ones that stayed hanging on withered.
I moved it into the shade of the hedge and kept watering it and spraying the aphids with home-made insect repellent, (basically a boiled conncoction made up of of two whole chopped garlic bulbs, and sprigs from anything in the garden which the aphids don't touch... rosemary, curry plant, thyme, mint, and lavender. It's foul.
Eventually it perked up, and new leaves have emerged. It's grown about another six inches, and I think it's going to make it.
Not sure how long I can keep it in a pot this size- and I'll have to find a way to safely over-winter it.