If you have their Solicitor's details, correspond 'only' with the Solicitor.
Solicitors charge for writing letters and reading the, too.
Write a letter refuting all of their accusations and, as I said up there^, invite service of proceedings *including their expert witness report* to be sent either to you (to keep your costs down), or to your solicitor.
I can almost guarantee that you'll hear no more about it.
The damage to the garden/lawn is a different matter