Acer care

We inherited a beautiful Japanese Acer when we moved house (Berkshire, UK). The tree is about 4 feet tall and is about 9 feet wide, it might be a Crimson Queen or Red Filigree. The first year we moved in the tree flourished. However as the tree lost its folliage last winter I suspected it was not regular loss. This spring 3/4 of the folliage has not returned and the branches have died. The leaves that have returned appear to be somewhat weak and there has been little extra growth, although there are watershoots on the trunk of the tree.
This might have been due to: -Weedkiller getting at the roots -Frost -A dry autumn/winter (2004/05)
There appear to be no signs of fungus or pest, bar ants. We also almost lost other shrubs and subsequently found out that the previous owners watered daily. So I guess that water is the most likely cause?
Given that these branches are dead what can I do to help the rest of the tree that is still struggling?
(pruning, treating the pruned branches, fertiliser, watering)
Many thanks
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