I think you've hit on the problem. You have done virtually everything right but it is the bottom of the hole that maybe is the problem. I would say that on clay soils you would have to dig the hole a lot deeper as it can fill up like a bucket as the water has nowhere to run. I would suggest that you lift out you acer (as it will still have its rootball) and dig the hole even deeper adding more grit. Once the tree becomes more mature it will deal with the extra water.

Many thanks Jimmy . . . just what I needed to hear. I will do as you say and then cross my fingers :)

I would apply a couple of handfuls of gypsum, and then drench it with compost tea. The calcium in the gypsum and the organic matter in the tea should open the soil structure. Long term, I would keep a shallow layer of compost and/or leaf mold on the soil, to keep the earthworms busy.
You might also want to double check that it's not planted too deep. The root flare--where the trunk widens out into the topmost roots--should just barely be exposed.

Sorry to disagree but not gypsum. Acers like slightly acidic soils and gypsum is calcium and an alkali and most Japanese acers are lime haters.

Thanks. We have added masses of grit, and ericaceous compost.

Gypsum is calcium sulfate, unlike lime, which is calcium carbonate, or calcium hydroxide. When gypsum dissolves, the solution is neutral to slightly acid, depending on the source and purity. In clay soil, the calcium ions bind to the clay particles, "flocculating" them, that is, causing them to stick together in sand-sized clumps. The leftover sulfate ions form a little sulfuric acid, buffered by the bound calcium ions.

Well Tug, whilst I would not use gypsum in my planting schemes, I have to say that you are never too old to learn something new. I quote: ''Applying lime to a soil will raise the pH. The other product we think of with respect to calcium is GYPSUM, a hydrated form of calcium sulfate (CaSO4 ). Gypsum is neutral in pH, and since it has no carbonate ion as part of its makeup, IT WILL NOT NEUTRALISE ACIDITY.''