Thanks Emery for your post.

Maybe I'll conside rmoving the plant to another area of the garden, trouble is that the soil is clay here, so I guess it doesn't matter where I move it, the soil is going to hold the water.

What is the optimum conditions for this type of Japanese Maple?

I have a different variety of Japanese Maple in my garden and it's about 50ft tall ) so perhaps the conditions are not too bad

Many thanks
Nick