Your acer needs to be planted in good compost rich soil not thin sandy soil. You also planted at a time of high temperatures and drought. Just how much water ddi you give it? It is unlikely to have been too much far more likely to not have been enough!

I suggest your fork a lot of good compost or organic matter in and around and that you also mulch with something like bark. I'd suggest lifting the tree and replanting but that will stress it even more right now.

As another thought did you remove the tree from its pot and soak the rootball for 24 hours or more before planting and make sure that the roots were not in a tight ball? And, if they were, tease them out?

Thanks for the fullness of this reply. It looks like this is not the best area for this type of tree! Sadly the "instructions" that came with the plant did not mention the actions of your last paragraph, so I did not do this. I had always thought that roots should be disturbed as little as possible when planting out. I will try watering more freely and see if I can get some of the compost out of the composter to put round it.
Thank you.

Unless you buy your trees from a private nursery it is rare to get proper instructions as to how to plant. Before we learnt to soak the root ball and tease out the roots we would wonder why, lifting a tree that was not doing well, its roots were still tightly wound together. The poor thing simply can't untie itself! The same goes for any pot grow plant you need to check the roots are not so tied up together they can't grow outwards.

Thank you for this extra information - I have mulched with grass cuttings and am watering more frequently, and already it is looking a bit better. Should I try replanting and adding compost when the weather is cooler, or let it continue to establish itself?

I'd keep mulching and see how it does but keep teh grass clippings away from the trunk of the tree.

Thank you - will do