What to do with issues with neighbours rented out property where a tree is pushing their wall onto our property and will eventually break

Our neighbours have rented out their house and a tree at a back of their lot is pushing their wall onto our lot, it looks really bad at the moment but will probably not break for another year or so.

We talked to the tenant and he said he would tell the landlord but that was a four months ago and no action. tenant is a realtor and there is no management company involved.

I was planning to send a registered letter addressed to the owners of the property but wanted to see if there were better options or if our council could intervene. Ideally I want to maintain good relations with the neighbours.