We own half of what was originally a detached house. The other half has been divided into four flats and I am constantly having to unblock their drains which cross our garden. The owners of the flats say it’s nothing to do with them and the freeholder has ignored my letters and phone calls.
The legal position will probably be that the owners of the flats are jointly responsible for maintenance and must ask your permission to come on to your land to carry it out. Ask a solicitor to double check this, but the chances are that you will have the difficult task of forcing them to accept this responsibility, perhaps by presenting them with a series of bills. Try to arrange a meeting with the four flat owners to discuss the issue.