A week is not long enough for possible problems in the soil from the Acer to be affecting your plant, especially if its surrounded with ericaceous compost. If you dug it up now, you'd find its still the same pot shaped rootball it was when you planted it last week.

Two possibilities - when you bought the plant, was it under cover, that is, under a roof of some sort at the garden centre? If it was, then it might be reacting to being fully exposed to the elements for the first time. Otherwise, it'll be watering - did you give it a really good soaking when you planted it, and watered again, another good soak, since, especially if its sited in a sunny spot?

Also, was the stump of the Acer fully removed? Chunks of rotting wood underground aren't good for any plant.

I would agree with Bamboo. Many garden centres 'buy in' their plants, especially from nurseries where the plants have not been hardened off. I know it's after the horse has bolted, so to speak but always best, this time of the year to leave in the pot for a couple of weeks, putting them out on warm days and bringing them in at night. Having said, I would leave the plant where it is now as moving won't improve matters.

Thanks for replies, I bought the plant direct from a nursery by mail order, it had recently been re-potted into a larger pot [they said]. After I received it I left it in the pot out of doors for 5 days before planting out.
I did give it lots of water after planting and 2 days later we had very heavy rain all day and night, I have since watered it again.
The dead Acer was completely removed and there were no roots in the soil when I planted the new plant - [I was never able to find out why the Acer died]
Maybe it's 'in shock' and hopefully it will perk up in time, fingers crossed.

I'm interested to read that your Hebe is doing well. Hebes need alkaline or neutral soil, so I suspect that your soil is one of those and is therefore not suitable for acid-loving plants, which is why you lost the Acer. Simply adding ericaceous compost to the planting hole in unsuitable soil isn't the answer for acid-loving trees and shrubs.

Well, that's not strictly true about Acers, Rosierose - japanese Acers dislike lime, but are fine in neutral or acid soil - other acers are not as fussy about ph. Japanese acers often get die back, and that eventually kills them; without knowing the symptoms and which variety of Acer, its hard to say why it might have died, but if it was a japanese variety, my money's on dieback...