[Company/ supplier address]
Dear [Name/house number/property name],
During the last [period of time] I have noticed the following problem [describe problem on your property], which has been caused by the roots of the trees in your garden.
Further to our conversations on this matter I have instructed an independent surveyor to investigate. [Or informed my insurance company who have commissioned a report].
The report has confirmed that the roots of your trees are to blame for the issues on my property. I enclose a copy of that report.
As you are legally responsible for the damage to my property, please let me know within the next 14 days how you will repair the damage caused, and what steps you would propose to prevent further damage.
You are welcome to come and inspect the damage and I will allow your contractors access to my property for this purpose and to carry out a repair. If you do not arrange for the repair to be undertaken within a reasonable time, I will arrange to have the work carried out by my own contractor and will look to you to pay the cost of these repairs in full.