Question
I have cut the tree right back to nothing but it has come back and the problem is the roots more than anything which are invading the rest of the garden. if i kill the tree ( if possible ) is there a way of rotting the roots without causing damage to surrounding pants/grass etc.

Answer
Here in the U.S. there is a herbicide called Roundup. This is an excellent herbicide to control unwanted vegetation. Sounds as is the tree has sprouted back out after you cut it--if so, this good for the use of Roundup. Spray the foliage with the Roundup solution--the tree will transport the chemical into the root system killing both the leaves and the roots. Be aware that Roundup will kill any green plant it is sprayed on so be careful not to get the solution on anything not wanting killed (spray only on calm days with no wind). Roundup can be purchased at most garden type stores it comes under other names also, look for the active ingredient Glyphosate on the label.