This is the advice we got in a similar situation.
Dangerous Trees
If you are concerned that a tree is diseased or damaged and poses a danger due to having fallen, or being at risk of falling, you should contact the owner of the land on which the tree is growing. If the land belongs to the local council, contact them to request that the tree is cut down or pruned.
If you are unable to contact the land owner or they refuse to take action, contact your local council's Environmental Health Office. The owner is not under any legal duty to take action, but will be liable if a tree they knew to be damaged caused damage to Property or injury to a person.
As prevention is often better than waiting for damage or injury to occur however, the Environmental Health Office may be able to invoke the Local Government (Miscellaneous Provisions) Act 1976 if the tree poses an immediate risk to Property or people. This allows them to serve notice on the land owner to make the tree safe. If they fail to do so, the Environmental Health Office may undertake this work themselves. The land owner would then usually be charged for any gardening required."