Whoever owns the land on which the trees are placed is responsible for any damage in common law negligence and any nuisance in common law nuisance.
If the owner of the road is not known, then it’s impossible to bring a claim against anyone.
If the wall is dangerous you can ask building control on the Environmental Health Department at the local authority (different councils have different departments dealing with this) to serve a Dangerous Structure Notice on the owner of the wall and it will be their problem finding out who owns it.
Can I clarify anything else for you?
I am happy to answer any specific points arising from this.
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