I had a gas leak outside my house about 2 months ago. It was a bad leak. My eunonymous which is about 4 feet away from where the leak was found is now dying. This is a very large plant - about 10 feet (3 metres) across and 7 feet (2.2 metres) high. It has leylandii on either side, same height, but they do not appear to be affected as yet. Could the gas leak be the cause of this. About half the plant is dead.
Unlikely that a gas leak will affect any plant. Are there any spots on the leaves of the healthy parts? If there are brown spots with grey centres, it could be Anthracnos, a fungal disease. Treatment for that is to remove all affected leaves and branches and spray with a copper-based fungicide. It could also be being killed by the Leylandii roots robbing the soil of moisture and food, effectively starving it to death.
I have checked the plant for Anthracnos and the leaves are just turning yellow and then dropping off. As you say it may be starving, i have given it a really good watering and a feed in the hope that it may survive. The plant has been there for about 30 years and the leylandii about 10 years. thanks for your help.