Why are the leaves on my Acer turning brown and falling off?

if it has got too wet that will be the cause. you need to dig in some grit and humus to improve the drainage in that area. did you dig a large hole for it or was the hole the same size as the root ball? if yes then i'd be tempted to lift it and redig the area to twice the original size, incorporating grit and humus and then re-plant.

Thanks for your help, i did only dig a hole slightly bigger than the rootball.
When would you suggest would be the best time to lift it?

No time like the present Bluedream! it will give it a chance to settle down over the winter....Good luck!

yes i'd do it now so it has chance to make new root growth before autumn. dont worry if it drops all its leaves though, that is a normal response.

Please remember to dig in lots of organic matter that said once the acer reaches the clay it will still have problems. I don't agree with trees in containers but in this case it might be your bet bet, especially if that area is always wet.

Well thanks all for your help, i think as my garden has a solid layer of clay about 12" down, i think if i leave it in the ground it will always be prone to getting waterlogged, so think i will maybe take Moon Grower's advice and transplant back to a large pot.

Thanks again for all the replies :-)

Probably wise given your soil and the wetness of your ground, good luck and remember to feed and water regularly.