If they're going brown and frazzled looking, that'll be too much wind or too much hot sun. I don't know where you are in the UK, so can't estimate how much sun you might have had, but certainly wind will cause this too. Find a sheltered spot out of midday sun for it. Also ensure you keep it well watered if you've recently planted it out.

Hi,
I live in Bristol and have 3 very beautiful Acers growing in large pots, I have ben growing them in these and in the same place in my garden for 3 years and I have found that this is normal to happen at this time of year.
My are now like that, The leaves will eventually drop leaving bare stems, do not prune it at all and next year it will come back even mor gorgeous. Hope this helps

Marion, it is not normal for an acer to be dropping it leaves this early, or going brown and frazzled. Our acers are in full leaf aside from one that got slightly wind burnt when we had wind from an unusual direction. I would not expect the acers to lose their leaves for at least another month. Bamboos advice is absolutely correct. I would suggest you look at repositioning your acers in semi shade and out of the wind Marion.

Not to mention that, if they've not been repotted in 3 years, they must be desperate by now, Marion...

Thanks for your comments I did not know how to upload photos on this bit so I have added one to the question page.